Chris,
I think you're right about the faintness of the Rosette nebula combined with the insensitivity of regular DSLRs to H-alpha.
Are you imaging with a bright moon in the sky, or from a place with lots of light pollution? The example you posted has a very bright skyglow despite the short exposure.
Am I correct in assuming that you have an f/4 imaging scope and an unmodded SLR? If so, I think you're going to struggle to see much/any of Rosette unless you're in dark skies and use very long exposures.
Here are a few pics showing what I mean (links to full quality versions of the attached pics):
1. 50% crop of a single exposure straight-out-of-camera taken under very, very dark skies: Rigel on the left, Orion's Belt left of centre, and Rosette on the right. The settings were 300 sec, f/2.8, ISO 1600 on a stock unmodded 5DmkII.
At f/4 (i.e. on your scope), this would be the equivalent of a 600 sec (10 min) sub at ISO 1600. Notice that despite exposing for 24 times longer than in your example, the Rosette is still quite dim here.
2. Same as in #1, but histogram stretched and +30 saturation applied only to the red channel. Just going by eye, it looks like both the Horsehead Nebula and Barnard's Loop are more brightly exposed than Rosette with the same settings.
3. Closer view of the Horsehead with a 59 sec, f/4, ISO 6400 exposure straight-out-of-camera - it's the equivalent exposure value to 236 sec (~ 4 mins), f/4, ISO 1600. Note from the above shots that the Horsehead was picked up by the unmodded sensor more brightly than Rosette...
It's tough with an unmodded SLR!
