Adding a custom property to a dimension

I have a location that I call out as "1.375, 24 PL", (this is not a hole, so hole call out won't work)

Right now the 24 is manually typed in. What I would like to do is link the 24 to the models custom property "slot qty".

I have tried

<DIM>, $PRPMODEL:"SLOT QTY" PL

<DIM>, <$PRPMODEL:"SLOT QTY">PL

<DIM>, <NUM_INST> PL

Is there a way to do this using SolidWorks 9 Service Pack 4.1?

Thanks

Misty