Couple thoughts:

- 29" frames and smaller riders don't mix that well. They do make bikes in that configuration but you end up with a bunch of compromises (like a heavily sloped top tube, minimal frame bag space)
- Straight top tube on a mountain bikes aren't common anymore for a bunch of reason. People are pushing the limits like crazy on bikes these days and you would be testicle free with such a high top tube. I've smacked my boys a few times more than I could count from running a slightly too big for me frame in order to get a bigger triangle for a bigger frame bag...
- Plenty of frame bag options these days. There is almost a supply glut of high quality custom bag manufactures out there. Custom made stuff is reasonably priced if your design is simple, maybe 10 to 15% more than an off the shelf high quality BP bag.
- Try running a frame the next size up. I do that myself on my Surly; I run a set of Jones H bars with a very short stem in order to ride a bigger frame.
- Look at top tube bags. Looks like on your current bike you've got piles of room to put a huge bag between the seat post and the top tube.
- Personally ride a Surly Ogre as my 'normal' touring bike. 2.5" tire clearance, handles somewhere between a MTB and a touring bike (good on/off road, decent ride on trails). 80 mm suspension correction/non tappered forks is kinda of a negative but I live with it. Steel frame to boot. I've slapped a 4L water bag in the frame bag, along with a few parts and tubes with a big of room to spare.