Other than using a workaround, you can't.
//var %file = $insong, %pos = $insong.pos | splay stop | bwrite ... | splay $+(",%file,") %pos
Pausing also might work, i'm not sure.
//splay pause | bwrite ... | splay resume
Edit:
You can delete threads 2 hours after creating them, so there's no need to make a new post telling people you've solved the problem or not to bother helping you.