How many square feet of floor do you have available for the radiant floor- I'm assuming it's not the full 12' x 6' (or is it?) Give yourself at least a foot of clearance around the toilet, since you shouldn't be heating it, and don't count anything that is under a floor-mounted cabinet, etc., just the raw open square footage.
And, what is the floor stackup starting with the underside of the sub-floor. Materials & thicknesses please.
Is the existing baseboard is all ~600BTU/foot @ 180F type fin-tube? If yes, the existing 6' of fin-tube puts out about 160-170BTU/ft, or ~1000 BTU at your water temps. You can then work backward from there using the output of the radiant floor & towel rack specs.
With the material stackup you can estimate the R value of the floor, and from there the BTU/hr per square foot you can get out of the using extruded aluminum heat spreaders at your ~115F water temp using the nomograph on p.4.
Say your actual free floor area works out to about 50 square feet, and it's a subfloor of 3/4" plywood with thinset & ceramic tile above it. The plywood has an R value just shy of R1, and the thinset & tile add up to another ~R 0.6, call it R1.5 for the stackup. If you follow the R1.5 line on the nomograph in that other document to the 110-115F range you'll see it will deliver about 15-17 BTU/hr per square foot, so the floor will be good for 50' x 16 BTU/hr 800 BTU/hr.
If the room is reasonably balanced with the rest of the zone at 1000 BTU/hr that means that if the floor is delivering 800BTU/hr, you can find a towel rack that puts out ~200BTU/hr @ 110F and be rid of the baseboard. The specified BTU output of radiators (including hydronic towel bars) is at a nominal 180F average water temp- the output at your water temps will be about 30% of that number. So a towel bar that's rated at about 600 BTU/hr @ 180F AWT (give or take a hundred or so) would be about right. If you preferred a bigger towel warmer, you could cut back on the amount of radiant floor, but if it's currently balancing temperature wise with the zone you want to keep it the total floor + rack to run between 900-1100 BTU/hr @ 110F AWT.
Other heat spreader types will have different output, and the responsiveness & consistency will vary. Extruded aluminum heat spreaders are pretty good- more expensive than the stamped sheet metal, but for the amount of floor area you're talking it won't break the bank. You're looking at probably 3 joist bays 12' long with a pair of runs per bay for a total of 3 x 12' x 2= 72' of extrusion, at about $2/foot.
Running it as a single run of 75-80' of half-inch PEX adds quite a bit of length to the zone loop, and it's advisable to set it up as a pair of equal-length loops with 2 x manifolds. The 6' of 3/4" baseboard has miniscule pumping head compared to 80' of half-inch PEX.