The ground channel is still there, fighting against the external ground (EG). Worse, you've increased the EG impedance by 6.49k because the IG-OG short comes from after R1G. This puts EG at a severe disadvantage to OG, which has a near-zero output impedance.
If you moved the left side of that short to the other side of the resistor, you'd still have the ground channel components there, fighting against the external ground.
It may be possible to hack the board to physically disconnect the entire ground channel from the circuit at the same time that you connect IG to OG, but I think it would require modifying traces. It's probably even tougher than you initially think due to the internal ground plane.
A middle ground that might work is replacing that SPST with a 3PDT. Use the extra two poles to open/close the R4G and R7G paths. This would still leave you with BUFG's output fighting with EG, but it wouldn't have a feedback loop wrapped around it to drive its impedance low. With the IG connection moved to before R1G, that might be enough to let EG win the fight for control of the OG node.
EDIT: I don't mean to suggest that you go and redo all that wiring to add a switch. You should first try simply desoldering R4G and R7G. This should be easy, since you've used SMT resistors. Open the preamp switch and try it. If it starts working well, you can consider how to proceed.
Edited by tangent - 4/9/12 at 3:11pm