I think that that is a good idea. If you have only been printing with MK or PK and the printer begins to sit with one black unused - then it begins to solidify above the exchanger. On the other hand if you are having an issue with the print channel itself (where both MK and PK print through) - this still can work. It may take several black ink changes and sitting to clear - but its worth the try and it will preserve the inks that are in the remaining positions. You may need to do powerful paired cleanings to bring the PiezoFlush to the black channel - but do not do too many in a row. The INITIAL FILL is always the best route of pulling PiezoFlush through the print head - and you might want to use that as a last resort in this case.
Please let us know how the black ink changes with PiezoFlush installed in both the PK and MK work for you. Once you restore to ink - you will need to do a number of black ink changes to restore to black inks again. Unfortunately, we have not tried this and we do not know how many black ink changes will be required.