I have not tried, but I did read of someone who converted a MicroVAX 3100 Model 20 to an Infoserver 100 by swapping the ROM chips. You also need to set the default boot flags to D0000000 so that it will boot from SYSD. I think the other oddity was that the console ends up on the printer port rather than the console port.
I think the model 20 is the only one that can be converted to an infoserver.
I agree that infoservers (especially the Infoserver 1000) are very handy and whilst OpenVMS 8.2 onwards can serve LAD disks, if you want to support VXTs a real infoserver is still the only way
I believe HP decided not to implement the particular functions required by VXTs for remote pagefile or the VXTLDR mechanism etc. It is not suprising as VXTs have not been manufactured for nearly 15 years and I guess they decided no-one would still be using them. I do know a few people at work however that refuse to let them go
It is possible to load them in 'host-based' mode from a modern VMS system using LANCP or DECnet, but you don't get the nice workgroup/centralised management features of the 'infoserver-based' mode and the VXT must have at least 12MB memory.