Yes, you can hide the Sirius unit. If the Tom Tom is what you end up staying with, and you can find a SIR-ELC1 or whatever replaced it, they aren't very large and can be hidden - probably even in the same area where I hid my interface which is below the glove box. There's a removable panel there that already contains a few of the T4R's computers that has more room left for stuff like this. All you'll have to do is connect the tuner to the head unit, mount your Sirius antenna and route its wire to the Sirius tuner. The tuner draws its power from the factory harness. The tuner then is controlled just like your radio, including preset ability, and the sound comes through the radio via the control DIN cable...and the sound quality rivals or exceeds a direct audio (RCA) connection's quality IMO.
I was also able to hide my plug and play, in the upper/hidden glove box, by laying it on its back. Works great, you can't see it, and I can still yank it out for use in my Tundra, house or workshop or whatever.
In fact, I highly recommend going the route of hidden Sirius tuner (of whatever style, fixed or plug-n-play), with a factory-style interface to the navi-system. Having all your audio controls in one place is very nice. To the unknowing, it will appear that your T4R just "came" with Sirius or XM from new.