Most coils can provide only limited heat gains or relatively small volumes. Now, some commercial units could provide what you need, but you'd have to look at the specs. Also, if the coils haven't been demineralized, they may not be working at their optimum (depending on the system, it might need to be done annually).
A small electric WH would run out of capacity relatively shortly (you only get maybe 80%, so 24-gallons on what you were considering), and then you'd be back to the same situation as it not being there at all until usage stopped, and the electric WH could catch up.
An indirect is more reliable and, if sized properly, should be able to provide ample water as it would have the entire boiler to recover with (assuming you ran it as a priority zone - may not be necessary but would give the most output and fastest recovery). Depending on your gas costs, it typically is a lot less expensive to do an indirect verses a properly sized electric.