In the Gimp GUI, the QuickMask is very useful for many things, but this functionality doesn't seem to be directly available through script-fu. No obvious equivalents were apparent to me in the procedure browser.
In particular, putting the (value/gray) pixels of a layer into the selection mask is the basic thing I need to do. I tried using gimp-image-get-selection to get the selection channel's id number, then gimp-edit-paste into it, but the following anchor operation caused Gimp to crash.
My other answer contains the "theoretical" way of doing it - however, the O.P. found a bug in GIMP, as of version 2.6.5, as can be seem on the comments to that answer.
I got a workaround for what the O.P. intends to do: paste the contents of a given image layer to the image selection. As denoted, edit-copy -> edit-paste on the selection drawable triggers a program crash.
The workaround is to create a new image channel with the desired contents, through the copy and paste method, and then use gimp-selection-load to make the selection equal the channel contents:
The functions that need to be called are thus (I won't paste scheme code, as I am not proficient in all the parenthesis - I did the tests using the Python console in GIMP):
>>> img = gimp.image_list()[0]
>>> ch = pdb.gimp_channel_new(img, img.width, img.height, "bla", 0, (0,0,0))
>>> ch
<gimp.Channel 'bla'>
>>> pdb.gimp_edit_copy(img.layers[0])
1
>>> pdb.gimp_image_add_channel(img, ch, 0)
>>> fl = pdb.gimp_edit_paste(ch, 0)
> >> fl
<gimp.Layer 'Pasted Layer'>
>>> pdb.gimp_floating_sel_anchor(fl)
>>> pdb.gimp_selection_load(ch)
Using QuickMask through the User interface is exactly equivalent to draw on the Selection, treating the selection as a drawable object.
So, to use the equivalent of "quickmask" on script-fu all one needs to is to retrieve the Selection as a drawable and pass that as a parameter to the calls that will modify it - And to get the selection, one just have to call 'gimp-image-get-selection'