How come this isn't a thing that someone makes yet? It would by nature also be an angled foregrip, so it would be kosher on pistols, Unless I'm missing seeing where they're being sold.

Not a Foregrip but Pistol Grip Glock extra-mag holder.

Tri Tech Tactical

Pistol grip capable of accommodating an extra Glock magazine.

TPG-ARGL910, is the shorter of the two options. It is designed to hold a subcompact magazine such as those used in the G26 and G27 pistols. The larger of the two products, the TPG-ARGL915, is designed for both the compact and full size Glock models. This should include sizes like the G17, G19, G22 and G23.


ProMag Vertical Forend Grip AR-15 Fits Picatinny Rail Polymer Black
The ProMag vertical grip is a full-size pistol grip with a picatinny rail adapter. The grip is constructed of lightweight polymer and the mount is steel with a black phosphate finish. This vert grip attaches with a large knurled cross bolt for easy installation or removal.


A.R.M.S.® #23™ QUICK DETACH THROW LEVER® PISTOL GRIP
The fastest attaching and detaching grip on the market by incorporation of the Throw Lever system. Engineered to be as robust and sturdy as the M16 lower receiver pistol grip. Optimum handling angle for a natural grip in a stabilized forward position. The #23™ will fit the A.R.M.S.® S.I.R.® System as well as any other hand guard incorporating Mil-Std. 1913 Rails.



GMF-G Glock Magazine Frame Picatinny Attachment

They already make one for $16
https://store.israeli-weapons.com/products/glock-magazine-frame-picatinny-attachment

What I'd like is a mag holder that attaches to a rail but holds it parallel. So you could lay a magazine down across the top or side.