Most of the Insteon switches will transmit their state when they are manually toggled. At least mine seem to.
So, if you get the "relay" type switch, and have it control the outlet, then turning it on and off manually *should* also send that on/off command to any other X10 devices that share the same HC/UC.
However, I've heard that some of the newest Insteon modules have removed X10 support. In that case, you might be somewhat out of luck with that particular solution.
A few years ago, I heard of a similar situation, where the user replaced the outlet with a SuperSocket (x10 controlled outlet), and wired it directly so it was always on. He then took the now-disconnected switch, and ran low-voltage wires to a PowerFlash module (also discontinued, from what I hear), and used that to send an on or off to the outlet.
It is a bit of extra work, and I'm pretty sure there are some code issues with doing that (unless the power to the outlet doesn't go through the box with the switch anymore).