I've been lucky not to have had any wasps in the house all these years, because there are lots of them around - and because other 'not-so-nastys' have squirmed their way in......to their demise, of course!
I've seen a large dog react to a wasp sting in the paw - it wasn't pretty, so I would really...really not like to see one of the cats go through it.
The dog 'went ballistic'...pain vocalizations that I'd never heard before, and running every which way. I had Benadryl liquid on hand and managed to get a few tablespoonfulls into him.
I really can't imagine trying to get anything into a cat in the early stages of a sting reaction...after a period, possibly, but with help.
The problem with oral
administration of anything is the time lag between ingestion and absorption - the fastest action is to be had by injection. If you feel there is a real danger of a sting, I'd say talk to your Vet and get a short course (demo, really) on how to do that, then have some syringes and injectible Benadryl on hand. Otherwise, for oral administration, tablets would be best. The liquid is extremely bitter and you'll produce huge drooling.
Then, you may need to deal with the stinger...extracting it without more venom profusion...here's a link with a how-to do that: What to Do When Your Cat Gets Stung by a Bee
Here's the info for Benadryl dosing
(page bottom) Benadryl® - PetPlace.com