Is there any issue with attached boom stands being microphonic to your pounding on the keyboard? I would have thought that for some of the heavier handed of us (or those where the keyboard is so light it tends to bounce a bit), it might be possible that the vibration from our playing on the keybed might travel up the boom and into the mike.

Has anyone tried making a recording with the arranger sound turned off, and see if the mike is picking it up?

I have always used a separate mic stand, but if going this route, I might definitely try to make sure there is a layer of rubber between the keyboard stand and the boom stand. Or an isolated mic holder...