I was looking for a UV filter for Olympus Trip 35, and I see that its lens takes 43.5mm filters. Before I go shopping for a filter: has anyone tried using a 43mm (not 43.5mm) filter with this camera, and does it fit?
Nope, can't say that I've ever tried what you're asking. But, I also wouldn't recommend it. Your proposed solution would leave the filter lacking .5mm - so it will probably not fit at all.
I did some shopping for you and actually can't find a ton of filters in the size of 43.5mm. Seems to be an odd size.
What I'd recommend is for you to get a Step-Up Ring from 43.5mm to 46mm and then go shopping for 46mm filters. There are plenty of options to choose from at this, more normal, size.
If the manufacturer says "43.5", it usually means 43.5. I don't think there are many people out there using this camera, so your best bet would be not to experiment and buy something actually created to fit it. Maybe something like this: