Yes and yes. I just looked up the US800 manual, and the digital (s/pdif) output is buss 3/4. The digital input is channels 7/8.
You'll need something to convert the s/pdif to analog and back again to accomplish what you set out to do in your original post. Something like a Midiman Flying Cow or similar. I still think it would be easier to accomplish with a larger interface with more I/O's (to avoid clocking issues), but you could pull this off...
To use an outboard rack effect, compressor, etc. in Logic (using the Flying Cow as an example):
* Connect the digital output of your Tascam to the Flying Cow's S/PDIF input, then the FC's S/PDIF output to the Tascam digital input.
* Connect the Flying Cow's stereo analog outputs to the inputs on your rack device, and the rack's outputs to the FC's analog inputs.
In Logic:
* For Compressors or EQ's, set the output of the tracks you want to send to output 3-4. Create a new stereo track called "compressor return" or whatever and set the input to input 7-8 and output to output 1-2. This way, the original music track is sent not to the stereo buss, but to the compressor and the return is routed to the stereo buss. Voila! Your mix now has the outboard signal.
* For Reverbs, leave the output of the track that you wish to send assigned to output 1-2, but also set up a buss send by clicking the little gray box under "sends" and selecting the next available buss. When you do this, Logic will automatically add a channel for that buss called "aux". Change the output on this aux channel to output 3-4, then add your track called "reverb return" or whatever, with input set as input 7-8 and output 1-2. Now, the send level on your original channel controls how much signal is sent to the reverb, and you can mix the reverb's return with the original track.