Hello Blowfin, I've been a HW-19 owner since 1997.
Mine started as a Jr also. If you have mats lying around, and want to try one, there wont be a problem in doing so, as long as you remember, you'll have to remove the Rubber Spindle Washer. You can even try clampless with this method, it's up to you, and what sound you find you prefer.
As others mention, it will sort of defeat the design, and of course, the clamping system helps eliminate any slight warpage some of your LPs may possess.
The "VPI" theory, was that by coupling the LP to Platter, this would thus eliminate resonances. I suppose mats of whatever type, and material accomplish this to varying degrees also.
I myself also use the AQ Arm, and use the Baerwald method of alignment, the Mobile Fidelity GeoDisc with good results. Mark