The VRS750 should fit the bill when connected to the "legacy" 25 pin connector on the remote radio chassis.
It is a simple repeater that crossbands whatever band the XTL is to UHF or VHF, depending on the model you buy, they are programmable from either 136-174 or 403-470. There may be a 450-512 model too. They use the HT750 radio platform internally. It can be programmed for 275Mw or 2 watts output.
I have one for my Astro Spectra plus and am very pleased with it.
People like to use VRS units, or mobile extenders, when a portable radio won't make it to the repeater or system, but the higher power mobile will. Also vice-versa with RX.
For instance, MARCS in ohio is 800Mhz, but portable coverage is limited as it was designed for mobile coverage.
So, VRS type units (on 700Mhz) are in use. Of course, in this way the same radio can be used both on and off the system, but it will allow the user to be able to access the system with the HT when the HT could otherwise not get in on its own by using the higher power mobile.
-Josh