There is a reference on the internet to someone who did just that. There are 3 filters in the rxtx board. The low pass filter for tx, the bandpass filter for receive, and a filter on the output of the oscillator before the mixer. You can make the board multiband by, either making plug in boards for each band, creating a band switch with filters switched for each band, or making a board similar to the low pass filters on a linear amplifier to have relays switch the filters. I do have plans for a new enhanced design and will be working on it over the next year or so. I am hoping the new design will work from below 2200m to above 2m.
