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Phil L

wrote:
Just to explore options.

I would assume the exposed parts of a steel "I" bean would be
(typically) be boxed in plaster board then skimmed.

Could the vertical faces of the beam be plasterboarded and skimmed,
but the underside of the metal painted without being plastered?

no, the rust shows through.
The normal way of plasterboarding the underside is to use expanding foam as
an adhesive.
Cut the lengths to width etc and have a piece of wood (3X2 or similar) to
fit underneath the plasterboard it's full length so that it goes up square
and doesn't sag anywhere, apply a few thin lines of adhesive (foam) to the
back of the board, put it up in place with the wood underneath and prop the
3x2 up from the floor in a few locations until the foam sets.
You can put the p-board up wider than you need it and cut it off with a saw
after you've noggined the webs and plasterboarded them, affix skim beads,
plaster and paint.