After a bit of thought, I believe I've found a basically permanent solution for this. I use word replacer (not sure how to add links without just posting them, you can google it, it is in the chrome web store) with a bunch of rules to enforce 'they' as default. If you put rules for longer strings at the top they match first ('he is' to 'they are' at the top with 'he' to 'they' lower down, for example)

You will have to put up with some number mismatch unless you want to add a rule for every verb in English ('they puts'), but I feel that that is an acceptable sacrifice.

EDIT: another issue: If you are actually talking about pronouns, you will have to temporarily disable it for things to make any sense whatsoever, and it doesn't seem to have a way to disable it on a specific page unlike the service I was using it to replace, so you have to use the extensions screen in settings.

EDEDIT: Also, and this is bothering me enough that I might actually stop using this, is 'her' as a pronoun versus 'her' as a possessive. for example in 'Get to know her' versus 'I found her wallet'. The first should be 'Get to know them' wheras the second one should be 'I found their wallet', and I'm not sure what to do about that. If I find/build an extension which can interface with a list of english words with part-of-speech tagging and have rules like 'her'->'them', 'her <noun>'->'their <noun>', then that'd work, but as is it is bugging me.