I have 1 HD/DVR receiver , 1 HD receiver, and 2 SD receivers on one account for my home. I use 1 of the SD receivers in my RV. I didn't call and tell them this, I just did it.
I have a regular Directv HD dish at my site even though I only use SD there right now. After a certain period of time, DTV deactivates receivers that haven't been used. If you call the 1-800 number, DTV gets your phone number and matches it to the account and turn the receivers back on, without actually having to talk to anybody. Takes about 30 seconds.
So to make along story short, yes you can do what you propose.