Instead of just creating the DNS PTR by reversing the IP and appending ".in-addr.arpa" it should be possible to create custom strings with templates.

If you only get a fraction of a /24 you can get your reverse-zone delegated as described in RFC2317. This makes crafting your PTR a special case - eventually the PTR syntax is site specific

i.e. your network is 192.0.2.128/25 and you agree with your ISP to use RFC2317 method 1.
Your ISP will add the following RRs in his Nameserver:

128/25.2.0.192.in-addr.arpa. IN NS your.name.server.tld
128/25.2.0.192.in-addr.arpa. IN NS your-other.name.server.tld
128.2.0.192.in-addr.arpa. IN CNAME 128.128/25.2.0.192.in-addr.arpa.
129.2.0.192.in-addr.arpa. IN CNAME 128.129/25.2.0.192.in-addr.arpa.
130.2.0.192.in-addr.arpa. IN CNAME 128.130/25.2.0.192.in-addr.arpa.
[...]
255.2.0.192.in-addr.arpa. IN CNAME 255.130/25.2.0.192.in-addr.arpa.

Thus you will have to fill your hosts into the zone 128/25.2.0.192.in-addr.arpa. and the PTRs will have to look like
138.128/25.2.0.192.in-addr.arpa. IN PTR my-server.domain.tld.
254.128/25.2.0.192.in-addr.arpa. IN PTR my-other-server.domain.tld.

Eventually I'd suggest to allow people to add a "PTR-Template" to their subnet configuration where you have variables for the dotted quad, the reversed dotted quad and every byte of the dotted quad.
A template for the above might look like "<% @ip⁴ %>.128/25.2.0.192.in-addr.arpa" and would be configured on subnet 192.0.2.128