bringing barren site into cultivation

We had some earthworks done and now we have a barren site maybe 50 m². I should so something quickly because it is getting hotter by the day. I have much other work to do in the garden, so I haven't got the time to hoe everything and bring in some decent green manure. I thought of making some hills with mushroom compost and manure and planting pumpkins in there, but there are another six weeks until the last frost. Should I do something meanwhile?
On the long run I want a potato rotation there, with maybe beans, corn, broad beans etc. I don't know if I plant comfrey there it will disturb such a planting because I will never get rid of these. I have enough comfrey plants (that's too easy).
Further thought it might be even advantageous planting comfrey. Then the pumkins in hills, in autum hoe, the comfrey  dies back here, then plant broad beans and in spring potatoes. That would mean, that I will have permanently comfrey in that bed. The comfrey I have gets waist high. Would this outcompete beans or corn or whatever? Or would it bring toomuch nutritients into the soil for beans?

I don't quite understand your situation (elevation, latitude, climate).
When in doubt, clover, with inoculation if you've brought up subsoil.  Always nicer to use the machine from the earthwork to do the site prep, or to scrape stockpile, and respread topsoil.  I'd contemplate a mix red, white, and crimson and add lupine if it is the beginning of something long term, maybe throw in some poppy or daikon.  Maybe grow some chicken food greens?  I don't think you'd need to hoe particularly if you've just done earthwork, even a wheel hoe or scratching with a 3-tine cultivator, would give you some seed, soil contact.  Maybe a dusting of straw.

elevation is 1000 meters, latitud ca 35°, climate is cool (but in Australian terms) I would call that temerate as the max. night frosts are around -5°C.
My idea is to use all these comfrey plants, but I have not idea if it is good to plant beans or corn in comfrey and I'm aware that I won't get rid of the comfrey.

Max average day temperature? Rainfall?

Are you designing using permaculture principles? If so I wouldn't plant the comfrey until you've done a site design.

At the moment it is wet wet. But in Australia it goes back to dry and then it gets wet and then it gets dry and these cycles are about six or so years long.
This part of the garden will be one of my "fields" not a real field - a farmer would laugh 50m², but a patch were I grow our stables like potatoes, dry beans, corn and the like.

will you be putting in shrubs, vines or trees? if so, you should maybe plan those first and get them in as they take the longest to grow and put in any perennials that you have planned..as soon as you can, often at the end of the growing year you can find great deals on these items

No, potatoes beans and the like, no shrubs or trees.

Thats a really interesting idea of using comfrey as a permanent intercrop for a summer annual patch!  I would try different kinds of distrubance in a small existing comfrey patch before planting in (unleash the pigs?).  I wouldn't worry about over fertilization using cover crops.