It would be good if we could set a DHCP reservation as needed for certain SSLVPN clients.
There are a few feature requests open for this:
FBX-10223 - Ability to assign a specific IP per user via SSLVPN
FBX-4677 - SSLVPN IP assignment via DHCP relay
FBX-17414 - Ability to assign IP address to SSLVPN users from Extended Authentication Servers (like RADIUS)
-SSLVPN policies can be governed by user/group name, so if you're looking to lock down or allow access to something via a specific user or group, I would suggest looking at that feature.
WatchGuard Customer Support