sascha, I think your'e mixing up two different things:
1. If you open a laptop and change its internal wireless adapter the BIOS might refuse to boot. 2. Connecting an external USB dongle does not change your hardware configuration at all.
If you use a USB dongle the BIOS should boot fine as you didn't physically change any hardware on the computer. Trisquel should recognise the dongle if it is supported by a free driver (you might need to enable or disable the internal wireless for this to work).
