Long time lurker, first time poster.
I think I might have a solution to my problem but I'd like to get some peer review on it before I start work.
I'm remodeling a master bath (50sf, 10×5) in a 1966 ranch and would like to add some nice tile. I pulled up the 5/8 particle board underlayment to inspect the subfloor and found it has too much deflection. The subfloor is in decent shape, however the 2x6x1 1/2 tongue and groove boards span a post and beam setup by 48". Everywhere I looked said this is a no-go for tile and I agree. Since the subfloor spans under walls and such, I don't want to rip it out and start from scratch so I want to add joisting from underneath. Here's my idea:
1. Span a double hung 2×6 between the beams on both sides of the project area, 48"ish span.
2. Hang 2×6 joists 16"oc between the double hung 2×6 cross beams from step 1, adding construction adhesive to the top of the 2×6's. Span is about 60", perpendicular to the subfloor planks.
3. Screw the subfloor planks into the new joists from above before the adhesive sets up.
From here, can I get away with 1/4" ply underlayment glued and screwed with 1/4" cement board mudded & screwed to it if the new joists stiffen things up well enough? A flat threshold is really on my want list. I'm open to a sloped transition, however.
Any other "gotcha's" that I might be overlooking? I thank you in advance for reviewing my question. Please let me know if I left out any details that leave ambiguities in my setup. 
Best Answer
Yes add the blocking as you have in your diagram.
You should also laminate your 2x6 tng flooring with plywood.
Are you doing large format tile?
The typical requirement for tile is 1 1/4" thick plywood subfloor. If you have less than this you can have problems with defelection. A 2x6 is usually 1.5" nominal thickness though older material was actual dimensions so thickness wise you seem ok.
In your scenario, I'd add the blocking, laminate the floor with 3/8" plywood using construction adhesive and screws, and use a decoupling membrane if you can tolerate the increase in height in the bathroom.
If you can't tolerate that much floor height, add the blocking, see if you have any detectable deflection ( I have 5/8" plywood on 16" centers and bouncing on my floor a friend can see it move @~180 pounds ). If you still have deflection, I'd go with the 3/8 plywood lamination and a reducer in the door.
Good luck.