The 5500s (mipsfair II) are a hard target.  They have an undocumented
'intelligent' ethernet interface, DSSI, etc..

The 5400s would be easy in comparison.  IIRC, the 5400s have the same
motherboard devices (chips)  as the Mayfair series microvax3s.
NetBSD/vax contains all the relevant Qbus adaptor and Qbus device
support. You'd have to glue in the vax-derived  rtc code,  and use
the appropriate technique to drain the CPU write-buffers.
If the PROM interface is the same, this comes down to judicious
cut-and-paste and debugging.

In contrast, a port to the 5500s requires dealing with obsolete,
undocumented,  DEC chips, or ignoring them and substituting equivalent
Q-bus devices and drivers.  It would really be much easier if those
were debuggged on a 5400 first.

still, if someone   wants to either try, or donate a mipsfair....