I've done this a few times. Its great for experimenting with oak, dry hops, fruit, coffee, etc.
Just make sure you purge w/ 5-10 psi two or three times before racking (and then dont invert the keg in between purging and racking).
I would start with about 1/4 of the oak you planned to use for 5 gallons. TASTE OFTEN.
Also, you dont want to carbonate while its on oak. Adjusting carbonation or off-gasing before opening the keg will carry off some of that oaky aroma. Blanket with 2 psi CO2 and then unhook the CO2 valve after racking and adding oak.