Scott:
IMHO, an ER32 Collet chuck and collets would be a better investment than an adjustable mandrel.
As a case, in point, with closed end pens, the closed end mandrel system that Arizona Silohutte sells for all of the popular component sets fits nicely into a collet chuck with a 1/2 inch collet.
Additionally, should you venture into custom pens made without components, a collet is a "must have" tool.
A $5 mandrel shaft will fit nicely into the collet making a precise "adjustable" mandrel.
I have all 3 of the readily available ER32 Collet chuck systems. I use them for MANY purposes. IN FACT , on my "main pen turning lathe" the only time the collet chuck gets removed is for cleaning.
I'll bet I still have 2-3 adjustable mandrels, but I'm not sure I could find one, because they haven't been used in years.