How about this?
1. No chaotic acts committed (as at present)
2. No killing of lawful creatures (as at present)
3. No lawfuls killed by your pets (as above)
4. No pets may die
5. None of <list of lawful NPCs> may die (includes all lawful unique NPCs (except possibly the Dying Sage on death after talking, depending on whether we want saving him to be necessary), residents of Terinyo and Dwarftown (except possibly the Demented Ratling), and the Eternal Guardian)
Condition 4 addresses the "tame it and then send it to kill something stronger" issue, while Condition 5 prevents use of abuse tactics to get rid of lawful beings that the PC may want dead - indeed, they can't be killed by anything. Because they're uniques or residents of "safe" locations, their deaths should not happen. In the case of the Dwarftown demon, you must protect every member of the town, none may die to it. As such, many may choose not to do the quest at all, as it has a relatively weak reward (mostly ratling fodder) and a high risk of destroying the chance to get the platinum girdle.
As a result, there is still a way to deal with regular hostile Lawful beings, using natural traps, etc. This basically applies to non-Dwarftown Dwarves, Solars, Ki-Rins, Blink Dogs, White Unicorns, Royal Guardians, and possibly the Ratling Guardians. And, of course, any other new random Lawful beings that might be added to the game.
I don't mind allowing the PC to use "cheap tricks" (they're not all that cheap) to deal with such regular Lawful beings. So long as eating Blink Dog corpses is a chaotic act (if it isn't, it should be), it'll only be a way to save the PC's life, and I'm alright with that.