I just discovered by accident that these things exist, this appears to be the rolls-royce product...
https://www.amazon.co.uk/Crenova%C2%...hone+borescope
I've not tried one, but thought I'd mention it as I'm pretty certain that now I know it exists (more basic ones come down to about £10 / $15 online) I shall almost certainly come up with a reason to buy one and I imagine that most other engineers with aircraft roles can think of a good reason to own a handy pocket sized boroscope.
Apologies if everybody knew about these already - but it was new to me.
G