It sure sounds like a wierd arrangement. Standard ASi is no more "failsafe" than FF... and loss of bus power will stuff them both.
The usual way to implement a "fail-safe" solution is to design/select the valve positioners to be *inherently* fail-safe under all known scenarios (eg. spring-return-to-zero) so that in the event of comms failure the positioners themselves can act appropriately.
Sometimes people try to keep the plant operating in some fashion on comms failure (eg. to reduce downtime) but it is usually cheaper, safer and simpler in the long run to shut the entire process down. IMHO, putting in an ASi bus simply to backup a Fieldbus is a dangerous waste of the client's money.