> I'm aware that both Squid and Apache can be used as reverse HTTP proxys
> (not done it myself though), but can either act as an HTTPS interface to
> a plain old HTTP server?
Yes.
The ProxyPass and ProxyPassReverse directives specifically take URIs[1] as
targets. This means you can set up your HTTPS section in your config file,
and use an
HTTP:// URI...
Vic.
[1] As well as paths - but that's not interesting in this context.