I went back and looked at their recruiting over the last 10 years. To be honest, it is a mixed bag. A handful of 4* players (including NFL players Joe Thomas and Peter Konz), mostly 3* players, and some off-the-radar 2* players. They were competing with other BCS programs for quite a few players though.
I agree that OL is pretty hard to project to the next level and that teams are all looking for different things in linemen, but I think it is also fair to question Marshall's recruiting as well. In his last stint here (albeit very short), he coached under another system coach. AZ posted this list on his site recently, it shows some of the players Marshall helped recruit to CU:
Brian Daniels
Mark Fenton
Edwin Harrison
Kharylake Martin
Gary Moore
Clint O'Neal
Tyler Polumbus
Drew Shrader
Del Scales
Fred Staugh
McKenzie Tillman
Jack Tipton
Clint Werth
Outside of Fenton and Polumbus, really not much there. Keep in mind he inherited Gurode and Rodgers, anchors on that 2001 team.