Evening all,
If you bypass the bandpass filters in an Saorsat LNB, keeping the LO at 21.200G that would give you existing 20250 to 19450 coverage with high side mixing, and 22150 to 22950 with low side mixing. Disadvantages will include image noise due to the removal of the BPF, but just for a test its probably worth a try. ( I have a modified Inverto here with external LO input for exactly these tests). The IF stages of the Inverto's are pretty good, and provide gain from about 600MHz to over 2.5GHz.
For polarisation, due to the use of the septum transformer, if you drill that out, you are left with dual linear polarity, you would have to physically move one probe round the board by 90 degrees, possible but probably messy. For experimentation, you can use the typical 1/4 wave depolariser slab in the waveguide.