I keep AltGr+8 mapped to × for just such an emergency.

The Compose key works great for this (I use WinCompose on Windows). Most of the time I can guess the key combination. I think × is Compose, x, x.

'x' is a letter, not the times symbol.

But if you have to choose between '\*' and 'x', then 'x' might well be the best choice, no? :-)

If they had used ‘x’ instead, I would be scouring the comment for where the variable had been introduced.

Ok, use 'x' and write "(note that 'x' means multiplication)" instead of "(note that \* means multiplication, there doesnt seem to be a way to escape an asterisk)". More legible and shorter and 'symbol is already used for the purpose' and..

It is _a_ multiplication symbol. Why not use period ".".

Because there are a lot of those in the vicinity and it's very small?