I've taken apart transformers like that, before -- they use a special "spacing" tape, which is also permeable (plastic or glass fiber felt, as it were), to make each winding layer into a sort of cavity where the wire rests. This keeps it away from the inside face of the bobbin, meanwhile the insulating tape is able to span full width across the bobbin (because it extends over the filler tapes), giving full creepage insulation.
Varnish is also considered an impregnant compound, which eliminates creepage. However, because it's solvent filled, it easily leaves voids, so one must inspect the parts using suitable methods (hi-pot, AC test, partial discharge), which may be more expensive than simply making the design oversize.
Resin-coated wire, baked into a rigid form, is how MOTs are constructed. The voltage between any pair of wires is relatively small, due to the consistent layering and high aspect ratio. The creepage around the outside is sufficient, because the wire is triple insulated (enamel + resin + varnish; though I would not expect any of those three to actually withstand direct application of 2kV, so this is kind of specious) and the height of the winding is adequate.
Hmm, offhand I don't know of any rules for trading insulation against creepage. I think insulation always has to be adequate (thick enough to meet the rules, and void free), even if some amount of air gap is guaranteed to be present, otherwise it counts as nothing.
Tim