I have met people - even streetsleepers - who are DEEPLY offended at being offered lai see by strangers. I also think you overestimate the language skills of both the proud, working class elderly and many expats ...
Certainly for many people of a Certain Age, lai see is about face much more than about money. This is definitely the case for many (not all) of the working elderly. You'll find many of them share poky little rooms but have not even applied for public housing because "that's for poor people."
So ... if you want to give lai see to a working elderly person without hurting their pride, give them a stack more work to do with it, like another big box of whatever they collect.
If you've actually got to know them a bit, and exchanged pleasantries, then "This is for your grandkids' education" is a good line. But they'll still probably refuse a couple of times, just to be polite!