No – the standard steam ones rely on gravity to close the inlet, but you can use a stainless spring to do the job instead and have them in any position – tricky part is getting the spring strength just right. You can buy sprung valves for oil but im not aware of any for steam/water so you may have to make your own. The clack doesnt have to be next to the boiler, it can be anywhere in the feed line, cant you just move it away to where you have space?

Clacks do not rely on gravity to seal they rely on pressure. Anyway what you need is a inline clack. These are sold by all the usual model engineering suppliers.

Lawrie Lawrence once wrote in ME that he would not entertain clack valves mounted direct on the boiler, but didn't say why. I can only think he wanted to keep them cool, perhaps so that scale deposition was less. I've read somewhere that scale is deposited as soon as water is heated, not necessarily when it boils away, but this might be rubbish. we have very soft water where I live, so no direct experience.