There is no way to do this unfortunately. You're stuck with "unrolling".
In memory, the $GENERATE directive causes individual PTR records to be generated. This can be observed by viewing the zone file received by the secondary servers after zone transfer, which does not contain a $GENERATE directive. There is no syntax that allows you to selectively override the individual PTR records.
An alternative is mentioned in Chapter 8 of DNS for Rocket Scientists, which is to add a step of using
named-checkzone to parse out the $GENERATE directive and replace it with individual PTR records:
The $GENERATE statement is executed while loading the zone file and
will result in an expanded in-memory version of the zone being used by
BIND9 operationally while the zone file itself is unchanged. If you
want to see the expansion (you don't trust the outcome or you want to
edit it) the bind utility named-checkzone will allow creation of an
expanded zone file (including the $GENERATE directive(s)) which may be
used as a template (or skeleton) and subsequently edited. As a side
effect of using named-checkzone any unqualified labels (names) will
also be expanded to full FQDNs and any $TTL directive will be used to
populate individual RR TTLs. This may, or may not, be a useful side
effect. Assuming the zone file containing the $GENERATE directive is
192.168.199.rev (as per this guide's zone file naming convention) and the zone name is 199.168.192.IN-ADDR.ARPA then the following command
will output an expanded zone file to 192.168.199.rev.exp.
named-checkzone -D -o 192.168.199.rev.exp 199.168.192.IN-ADDR.ARPA 192.168.199.rev
The downside is, naturally, the fact that your zone file on the master becomes much larger. At this point you're only using $GENERATE to build the initial reverse zone for you so that the individual PTR records don't have to be typed out by hand, and a shell script could have easily achieved the same end result there.
This probably isn't the solution that you were hoping for, but that's the state of things unfortunately. :(