If you are using fluidsynth you can use an NRPN to adjust the filter cutoff -
fluid_synth_cc(synth, i, 0x63, 120); //NRPN MSB sf2
fluid_synth_cc(synth, i, 0x62,

; //NRPN LSB Select Filter cutoff
fluid_synth_cc(synth, i, 0x26, nrpndata & 0x7f); //NRPN DATA LSB cents
fluid_synth_cc(synth, i, 0x06, (nrpndata >> 7) & 0x7f); //NRPN DATA MSB
Dont ask me how it works but that is what I used in eplayOrgan. You may be able to use the CC directly.
csw900