Open Source version of MOST?
I doubt if there is an open source protocol that provides substantially similar functionality as MOST, mainly because there's little need for it. The companies who make MOST equipment don't need an open source protocol, because they have MOST.
Hobbyists, who might want to use open source resources, are not constrained by interoperability with other components in the vehicle, so they are free to invent their own protocol if they want some of the features of MOST. 

The MOST Cooperation does provide an online book that at first glance seems 
to reveal most of the details of the protocol. 

A simpler alternative to MOST, if you just want a point to point link for audio data, is S/PDIF and TOSLINK. Neither of these is open source, but there does seem to be substantial information online to enable designing interoperable components. If you look into MOST, you'll find it's basically S/PDIF multiplexed onto a packet ring network, with a bit of additional control information and low speed side channels added. 

Olin, answering your other question, mentioned the CAN bus, which might be more accessible to you if this is a hobby project, but also might not be as well adapted to multimedia data as MOST.