Why can't you grip the magazine itself? I've done that for years with my G26. If the problem is that it looks funny, then why not a +2 extension on a G19 mag? Both would provide extra grip area, both would be cheaper solutions than creating a new frame type (i.e.: the G19X), and neither would create a new pistol that is incompatible with G19s that are already in the .mil supply system.
All of the reloads on my belt are 17 rounders anyway, so I could give a shit about compatibility.