The problem at hand
I'm fairly proficient with the usual pathways for getting frost or fire or radiant damage combos/optimization going with regular weapons or using holy symbols, but I am new at trying to optimize damage for ki-focus implements. My usual method of grabbing a weapon that can apply a damage type to all attacks made with that weapon is out the window. Aside from the class itself being homebrew, the game is run RAW and any and all items and feats that I could qualify for are available. I just hit level 8 so I have a feat to spend and am also willing to retrain a feat as well (improved initiative) so I have 2 feats to work with and I have not multi-classed yet.
Currently I'm running a custom class for the game based on the Geralt of Rivia which functions very closely to the Executioner class in 4e. I have poisons/bombs that are daily use items that I craft in much the same way as the Executioner. The Witcher class is a striker with a heavy controller feature. My at-wills are all weapon attacks without any special keywords. I do have more regular (vs. essentials) encounter powers (based on the aard signs) which have the implement keyword. There is no common theme among the encounter powers as they all target different NADs and if they have a damage type, it is different from the others. The poison dailies are applied to my weapon and augment the damage of any weapon keyword attacks while the bomb attacks are made using and have the implement keyword. I have access to the Venom hand feats as part of the class and for the class has both the Martial and Arcane power source, so any feats simply requiring either power source (like Arcane Mastery) are available to me.
Current level is 8, but the campaign will go on through paragon and I think into epic (will need to check with DM). I am seeking to optimize for now, and onward. I'm looking for any feat and item combinations for implements similar to frostcheese or radiantcheese for regular magic weapons. Here's the character sheet as of lvl 7. https://skydrive.live.com/redir?resid=723A28155692C47F!554&authkey=!ANK_HRfu03BkkZo
Please asks questions in comments for clarification!
What I've Found
Using only ki-focus implements has a very limiting effect. They are one of the smaller magic item groups and very few of them have properties or powers that add or change damage types. Since I was looking for being able to permanently deal one type of damage, I've ignored ki-focus implements that only have daily or encounter use damage type changes/additions:
Of those, Thunder had the best feat support. There were damage feats for dealing thunder damage (2) and feat letting me slide an enemy 1 whenever I hit them with thunder damage (reinforcing the controller aspect of the home brew class).
Ultimately my solution was to stick with ki-focus implements because switching to any other implement type would have raised my magic item requirements. Using swords as implements would require having 2 magic weapons kept up to par and all other implements would require 3 magic items total (needing to use both a silvered and non-silvered sword to get the most of my class features).