T-Mobile say you have to access your account to start the PAYG period going - a certain element of chicken and egg here if you don't have fixed BB available at the time, so presume it is accessible by phone (although I don't know how this works if the user is not on the T-Mobile network for their phone mobile service).
FWIW if you can get T-mobile's 3G, you can almost certainly get 3's service. The 3 MiFi might be of interest if your PCs are wireless.
Both T-mobile and 3 allow you to connect to their control panel pages (e.g. My3) with no credit, so as to top up the account.
However do be warned if you leave the account for ages (6months +) all
providers reserve the right to disconnect the SIM and reallocate the number. You can get any credit back but you would need to register and spend time on the phone to customer services.
pro - on THFB - sync about 17.2mbps - BQM