Open source basebands cannot, legally, in most parts of the world be up-datable by the user, which removes most of the interest.
There are several good reasons for this.
Radio is a shared resource. Cellphones only work as well as they do as the towers arrange it so that no cellphone is transmitting on top of another one.
The modem hardware is quite capable in most cases of transmitting right over the top of other transmissions. The worst case would be a free app turning up that gave free data transfer between nearby phones. And did this by ignoring the towers, and going direct.
This has the potential to knock off dozens of calls from the network per user, some of which may be emergency calls.
FCC/... approvals are inherently with a given software version of the modem - most of the behaviour of the modem is set by software - and changing that software without approval will void the approval of the phone.
In some countries, there is actual specific legislation.
If your open-source baseband could change the IMEI, then once you have been informed that this has been done, you are actually committing an offence if you continue to sell the phone which enables the user to do this in the UK.