So, where has your diagnostic process already been? The obvious is to remove the inspection cover of the cylinder casing to see if it is the linkage that is siezed. A generous dose of lubricant and working would resolve that. Removal may be necessary and isn't an impossible task. Failing that, you need to drain the hydraulic system. Stuck clutch plates or seized cylinders don't tend to give an absolutely solid pedal as you suggest - there would usually be a little give. Indeed, stuck plates would allow you to press as normal with loss of pressure.
Let us know how you get on.