That's nice thinking outside the box...

I think the only issue you might have is, how well do the two sync together? Roland, in the BK-9 manual, state that the shorter a loop is, the better it will sync up. So if you are talking about say a 32 bar segment, you might have issues with it drifting against the MIDI clock. Roland also say you get tighter timing and better processor load (less load) if you use .WAV's or AIFF's, so I'd definitely recommend that.

But one of your main issues with your OP is that you can't record ONLY the vocals in the BK-9's recorder. It is going to pick up the backing too. So, if you still want to remain in arranger mode, but have backing vocals (or anything else you feel like) ONLY on the audio loop, you are likely going to have to record the song first (as a .WAV), transfer it to your computer DAW, then overdub the vocals to that, THEN save them as .WAV's there (and crop them up into different sections) and transfer back to the BK for assigning to the keys.

Seems like quite a bit of work to do...

Personally, I think you'd be better off recording the basic backing for the entire song and singing the backing vox (one part wouldn't need any overdubbing, so you could use the BK's audio recorder), then try my trick of chopping the song into its sections, and assigning THOSE to keys, then there would be no issues about drift (other than the basic drift I have yet to test), you play what you feel like on the top, and the BV's come in where they should without you having to trigger them.

But to trim the audio file sample accurately, and perhaps time stretch or shrink it fractionally, so it compensates for any MIDI clock drift (it might need to be 119.997bpm rather than 120bpm, for instance, to line up perfectly), I'd say at some point or another, you are likely going to have to involve your computer...

I'll be testing all this soon, so I'll report back how well the tempo's stay together over longer bar amounts. To be practical, I'd say that a 32 bar (maybe 16 is usable) section with unnoticeable drift is going to be needed.

Mind you, if you ARE hand triggering the loops, but they are only one shots (in other words, not loop), even if they drift off by a tiny fraction by the end of the segment, when you call the NEXT loop, it WILL start one the arranger's 'one' tight as a drum.

So, as long as you aren't expecting these long recordings to loop and stay in sync, you should be good to go.