You're right (sort of). Nothing specific about Mustangs and Camaro's, everything is allowed Subframe connectors in both SP and SM. However, they must be Longitudinal and cannot connect to more than 3 points on each side
Here is the verbiage from 15.2.E:
E. Longitudinal (fore-aft) subframe connectors (SFCs) are permitted with
the following restrictions:
1. They must only connect previously unconnected boxed frame rails on
2. Each SFC must attach at no more than three points on the unibody (e.g.,
front, rear, and one point in between such as a seat mount brace or
rocker box brace).
3. SFCs must be bolted or welded, but welding must be to the OE sub- frame stampings, not to the floor pan in between.
4. No cutting of OE subframes or floorpan stampings is permitted. Drilling
is permitted for mounting bolts only.
5. No cross-car/lateral/triangulated connections directly between the
driver’s side and passenger’s side SFCs are permitted. Connections to
OE components such as tunnel braces or closure panels via bolts are
allowed and count as the third point of attachment. No alteration to the
OE components is permitted.
6. SFCs may not be used to attach other components (including but not
limited to torque arm front mounts or driveshaft loops) and may serve
no other purpose.
So, I don't know the shape of your brace, but you could sever it in the middle (if possible) and call it a pair of 3 point subframe connectors OR as you mentioned, you could replace it with a widthwise brace between the two LCA's.
So I guess it would be worth mentioning that as it sits, with the brace, your car would technically be in F-Prepared or X-Prepared