Thanks for your insights!
I had a chance to inspect the Gitzo 2541 (the four section version of the GT 2351), and it is certainly as sturdy (if not more so) and much lighter than my older aluminium Gitzo 226.
I was also able to compare the GT 2541 to the GT 1531, and though there is not much difference in weight, I think that the 2 series would be better suited to MF (and LF in a pinch). The GT 1531 is however amazingly stable for its weight, and certainly compact.
Years ago, I replaced the center column of 226 with a short center post made by Kirk Enterprises, which provided a stable flat platform for my Arca Swiss B1 ballhead. I see that Kirk still makes a similar solution- "Designed for the Gitzo 100, 200, 300, 400 and 500 series tripods, these plate assemblies mount between the tripod and the tripod head to eliminate the original center post, thereby enhancing stability, reducing vibration, and allowing you to get closer to the ground":
Dead Link Removed
I think that a GT 2531 (with a Kirk FP-200N short center post) would provide stable support for an AS B1 ballhead (or a Linhof 3663 3-way levelling head) and would make for an excellent MF (and LF) travel or hiking tripod.