Which type of 28"is it?
The RF could probably work by connecting the HDR to the Vtape connector.
The biggest problem lies in the needed so called "tunerdump". All the channels on the avant has to be dumped in the HDR1.
Manually tuning on the HDR1 will work, but to program the HDR1 you have to use the HDR1 menu. It will not work with record menu in the avant.
The biggest chance do you have if you're having a type 8300 or higher with the most recent software available. With this software is possible to rename all the 3 scarts on the back.
You also have to disable (for a short period) the internal Vtape in the servicemode.
With the older type of the avant is it not possible to attach the HDR1 to the Vtape scart. This is due to the fact that the internal Vtape doesn't communicate through scart but through another internal connector. So the HDR1 will not respond to any instruction.
The only option you have is to attach the HDR1 to the AV scart as a Vtape2 and to programm it through it's own menu. But programming it with the help of teletext will not work. Also the record record function will only work on the internal Vtape.
As Graig mentioned the HDR1 does work perfect with a avant RF (DVD) without a Vtape. Any more recent type of beovision works perfectly.