The issues with putting steam baseboard radiation directly under/behind toilets are twofold:
1: The toilet can heat up to a very uncomfortable temp during cold weather (or during recovery from setbacks) when the duty cycles on the heating max out.
2: The overly-warm toilet causes the seal to the drain to fail (and the traditional wax seals can melt entirely!)
Even though it's traditional to put the radiators under windows (a practice begun back in the day of leaky single-panes, that collected copious frost/condensation otherwise), it looks like you have sufficient wall length between the door and the right side wall to put cast iron baseboard opposite the toilet & sink, maybe even turning the corner, stopping before it's even with the toilet. Whether that's easy or difficult to implement the way the current steam plumbing is currently routed under the floor isn't obvious. The two biggest vendors of c.i. baseboard are Burnham ( the product line is called "
BaseRay"), and
Weil McLain (called "
Snug".) The ~8"-9" tall versions have comparable output-per-foot to 2" fin-tube type baseboard, but since a large fraction of the output is radiated rather than convected (almost all fin-tube output is convected air) it's noticably more comfortable.
Heat loads of bathrooms are pretty low- the size of the radiation needs to be reasonaly balanced with the radiator sizes relative to their room loads on the rest of the system though so that you neither overheat or under heat the bathroom relative to the rest of the house. If the existing radiator is sized with reasonable temperature balance, you can measure it up to determine it's "
Effective Direct Radiation" (EDR) size which can be applied to the EDR of the new radiator or baseboard. It doesn't have to be exactly the same EDR as the original radiator but keeping the new radiator neither more than 1.5x nor less than 0.5x would be prudent, if it's working OK now.