An easy test is as follows:

runnas /user:JOE.USER /pass:xxxxxx cmd

then in the new command window if you do a SET you will see that the environment variables belong to JOE.USER and if you do a REGEDIT you'll see that the HKCU belongs to JOE.USER.

Could something like Chris suggest be done ? Kinda. The trick is to have runnas do two things:

1) Map the ENV variables of the calling process to the new process

2) Map the HKCU of the calling process to the new process.

What I mean by kinda is that your not really elevating the priviledges, your running the current user environment with a more priviledged account.