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Yeah, I've got the 57cm version so I lose a lot of triangle real estate compared to you.
I guess it's still an option for me but I'll need to build it up and have a look at how much room I have with bottles and knees and stuff before I proceed with a bag.
I used a Deuter frame bag during PBP which worked "ok", save for it not being secured at the bottom so it flicked left to right all the time.
Actually, I have the normal Kinesis 4S sitting at home without a rear wheel at the moment - I should just put that back together and test fit bottles to see how much room I have!
hippy-
You can test the leg rubbing also, I did a simple test with bubble wrap around seat tube and headtube and around again, with some tape to keep it in place while on the indoor trainer.
Looking at the geometry chart, you'll loose 1,5cm of height and 1,3cm of lenght, but if you use some 750ml bottles instead of 1L you may get even more space since Camelbak 710ml bottles are 4,5cm shorter than 1L SIS ones. Anyway any storage space gained will be great to remove heavy objects (spares, phone, battery...) from the seatbag to avoid some wobbling when out of the saddle or in hairpin corners.
I just left enough room for the bottles to be open without touching the bag, no mental adaptation required to get them in/out even going full gas in the 9th hour to make the 10h objective.
Talking more about framebags... I was intrigued what could I store inside a rear triangle framebag like the one shown here between the seat stays and the seat tube... since my frame is so big, theres a nice space there. About the same size as a fuel pod but with a piramidal shape.
bertocq
Took 1 week to get it made and 1h to cut the cardboard with the marks for the straps so he could use it as a guide. There must be at least a guy sewing custom made bags in the UK that can get them done in less than 2 weeks...
Don't know you legs measures, but mine pass about 2cm away from the tubes, and I'm not a skinny type (the one on the right) but neither a pistard/TT. While out of the saddle the fuelpod wasn't a problem either with rubbing legs, but we are talking framebags here.
I've used the Lezyne Flow Cage SL just because I wanted to use the SIS 1L Bidons and Mike Hall used that combination without problems in this winning Tour Divide (but with the expensive carbon version). I've done some curb jumps and fast descents with potholes with both bottles full and neither jumped out. Also its easy to really insert them sideways by squeezing them a bit.
The trick to make a framebag so long (frame size is 63cm) with will no bulge in the middle is to sew one or two velcros on the inside of each side, so when you link them the make a "bridge" between each framebag side, and also act as little walls to contain the cargo so it doesn't move around too if you take something out.
I'm sorry for the vertical video, but was the best option to show what we are talking about, apply cold beer to eyes afterwards.
https://www.youtube.com/watch?v=S-VnstDT4BQ
Just a stupid idea I had, to put a card inside with my personal & health data just in case someone finds me in a ditch... >_<. But @CarlosBI had a better one for his framebag: sewing some reflective stripes on the sides. We both used his cage dropper adapter on the vertical bidon cage to get more room for the framebag, still waiting for the screws to arrive from china to sell them in the online store.