The problem with that is that the pug has no guarantee of future loot other than your word. And just the fact that you are using loot council to give priority to a guildie over the pug will seem unfair, which will probably cause the pug to leave in rage when it happens, bad-mouthing your guild on trade, which will be bad reputation for your guild, sour taste in everybody's mouth, and eventually your raids not happening anymore / guild disband.

If you plan on keeping your guild reputation intact / ever pugging or recruiting in the future, respect your pugs. That single person may have only joined a single run, but your pool of pugs as a whole have been with you the entire time, and have gone through just as many wipes as your guildies to bring you to where you are. The pool of pugs "deserve" loot just as much as your guildies do.

Random mainspec rolls with lootcap is probably the best way to keep things fair to pugs while distributing loot evenly. Of course you may down 3 bosses and the three pugs win one loot each leaving none for your guild. Sh2t happens, sometimes you have to just deal with it. Or, like other people have suggested, increase your raid pool, extend invites to the pugs, and penalize the no-shows since they are the ones that caused you to pug in the first place.