I've had my keg for almost 9 years now and have never used anything but lump charcoal for fuel. I did a couple of butts last weekend and had leftover lump in the keg. I gave it a stir and put some Kingsford Natural Competition briquettes in a ring on top of the lump, probably around 20 of them. Lit it just as I would if it was all lump with a starter cube. Added some hickory, pecan and apple chunks for smoke. Set the vents just as I would if I was using all lump.
Well, everything started off fine, temp stabilized between 225 and 250* for the first few hours then started rising. I shut the vents down some and it was still higher than I wanted, around 280 so I shut the bottom vent completely and had the top around 1.5. I wouldn't expect things to burn hotter since lump generally burns hotter than briquettes. Everything turned out fine, these were some juicy ribs.