Would a water repellent spray make a jacket reasonably usable in a light rain?

It depends on the jacket.

Some clothes etc. are designed to be waterproof but only are if you spray the seams, for example.

Other clothes etc. are not designed to be waterproof, and spraying them won’t make them waterproof because the fabric just isn’t (unless there’s some hard-core spray I don’t know about).

I expect different sprays and fabrics work well together, but I would consult the clothing and spray documentation, or good sales staff (e.g. at REI) to figure out what works best on what.

You can spray a jacket with ScotchGuard, and it will be water resistant. I use to go skiing in jeans that had been Scotchguarded, it works great.

But it depends on what kind of jacket you are trying to waterproof, what the fabric is.

Outside problem is that the sweat and body moisture will be trapped under the jacket (think plastic garbage bag).
GORE-TEX® will repel water but let body moisture pass through the fabric.