While I haven't tried this, I think you might need to "SET UDFPARMS"

This controls the default way that parameters are passed within VFP. The default is by value (in only), but can be overridden in VFP by passing paramters in the form @param1. I assume this ability to override the default is why you see [in, out], so that it can act appropriately.

Setting the default to be by reference instead of by value, may do what you want.

Mike