As usual, the Lady from Philadelphia has to step in and resolve this issue.

It would be a simple matter (as someone else mentioned) to use single-pole,
double-throw switching at both ends to permit either computer control or
local control of the light. (This type of switching has been in use -- I'd
assume -- close to a century. You should be able to find a schematic in any
book on household wiring. It's trivial.)

The "catch" is that the switch positions no don't directly correlate with
whether the light is on or off. (I normally wire the switches so that when
both are up or both are down, the light is on.) So closing the relay
operated by the microcontroller will turn the lamp on or off, depending on
the position of the local switch. Do you understand that?

If this ambiguity isn't a problem, then simply wire up the system as any
two-way lighting system, and you'll have full control at both ends. You just
won't know whether closing the microcontroller relay turns the light on or
off.

Note that if you add a manual _override_ at the light's location, that
_forces_ the lamp on, then the microcontroller _won't_ be able to shut it
off. I don't think that's what you want.