Seeing Jan LA3EQ's recent 4km earth mode tests with WSPR at VLF I must pass on something I'd forgotten about.
WSPR software is able to transmit a WSPR signal at least to 22kHz just by selecting the TX frequency to the chosen frequency. For example, if you want to send WSPR at 8.970kHz just select the band as "other", set the dial frequency to 0.000kHz and set TX frequency to 0.00897kHz. Listening to the tone it definitely is at the frequency stated.
What I haven't determined yet is how high the WSPR software will decode at. When I tried WSPR some time ago on 8.97kHz I used my up-converter and FT817 as the receiver. When I tried to decode directly at 8.97kHz today (setting the RX PC's dial frequency to 7.470kHz) it did not decode.