[SalesForce] Any way to recall arbitrary approval requests from Trigger

Per this source:

When the assigned approver is a queue :

• Any queue member can approve or reject an approval request that is assigned to the queue.
• Approval request emails are sent to the queue email address. If the queue is set up to send email to members, then approval request emails are also sent to the queue members, except those whose approval user preferences are set to never receive approval request emails.
• When an approval request is assigned to a queue, each queue member’s delegated approver also receives an approval request email notification.
• Because email notifications to a queue aren’t intended for an external audience, any instances of the merge field {!ApprovalRequest.External_URL} in the email template are sent as the equivalent internal URL.
• Salesforce1 notifications for approval requests aren’t sent to queues. For each approval step involving a queue, we recommend adding individual users as assigned approvers, so at least those individuals can receive the approval request notifications in Salesforce1. To have both queues and individual users as assigned approvers, select Automatically assign to approver(s) instead of Automatically assign to queue in the approval step.
• When an approval request is rejected and returned to the previous approver and the previous approver was a queue, the approval request is assigned to the user who approved it instead of the queue.
• The Approval History related list displays the queue name in the Assigned To column and the actual user who approved or rejected the approval request in the Actual Approver column.

Although, this is a really late answer, I hope it would help someone looking for a solution to this problem. I came across this work-around in the same week this question was originally posted, but missed out to post it here. [Thanks to my co-worker Frank Dipoma who suggested this approach to me (not sure if he is on this forum)]

Per the standard approval process in salesforce, system admin or the current approver of the approval request have the ability to re-assign the request to another user and most companies don't fiddle over this standard functionality. However, the client that I was working with had a specific requirement where a standard user would update a user lookup field in an opportunity record and all the related pending approval requests should be re-assigned to this new user. The most obvious approach anybody would take is to put the code snippet (given in the question) in a class marked as WITHOUT SHARING, but this will NOT specifically work in case of approval request re-assignment (nor will any of the options mentioned in my question). Anybody interested can try this out.

Note that this workaround exploits a loophole in salesforce (I don't want to call it a bug or defect in salesforce). Since, approvals & approval re-assignments are quite critical for any business, I recommend anybody trying this solution to have a thorough understanding of the use-cases, before adopting this workaround.

Without getting into minute details of the work-around, I'll summarize the approach that worked for me. The idea is that the apex trigger will send the relevant details to a specific email service address and then the inbound email handler code will use the details from the email to process the relevant approval request records.

• Create an apex class that implements Messaging.InboundEmailHandler and in its handleInboundEmail method, place the code snippet given in the question. (Sample code for the inbound email handler class can be found in the developer guide here, although it isn't related to opportunity object)
• Create an Email Service Address and associate it with the apex class created as mentioned above. This can be created from Setup > Custom Code > Email Services > New Email Service. After a new email service is created, a email service address needs to be created within. Note that this set-up has other standard email related configurations like accepted FROM addresses, dis-allowing attachments etc. which may be opted.
• In the apex trigger (opportunity trigger in my case), send an email (SingleEmailMessage) with parent record ID (or opportunity ID) in the email body or subject. This email should be sent to the service address created above.

My memory is a bit hazy on a particular point related to this solution, since its been over an year. The apex code in the inbound handler class will run as the context user specified in the email service, but I don't remember whether the LastModifiedBy of the approval request is updated with this context user name or the actual user who updated the opportunity record.