Some, but not all, cappuccino machines have a boiler. the lid contains a safety valve and there are two 'take offs'. One uses steam pressure to force hot water through the coffe, the other is a steam jet for milk frothing. The former is no use to us, but the latter is a good source of steam at a fair pressure for small engines – just put a silicon pipe over the end.
They can cope with being boiled dry (having thermal cutouts) and have a design so that they cannot be over-filled, the only limitation is the relatively small water volume.
I have one of these boilers, removed from its case, the only reason I haven't done anything with it yet is the need to make an (electrically) insulated container that is also waterproof.
Neil