It depends on the intended balance of what you'll accept as in-universe or not, really. People here seem to not like physical-looking barriers that aren't, and then again how much 'glow'-style UI-only stuff do you want that's 'clearly' not a true feature that a dorf would be faced with in a first-person viewpoint.
A far earlier attempt at mocking-up that I did, pre-Steam, experimented with little "L"s and/or "triangular plates" at the corners (less than a third of the tile-side, so that a 1x1 didn't look like you could tightrope across the 'spar', nor obviously stride across the gap). In the above I nearly left in the opposite side 'footing' in that posted one (the 'plaform heel' I put under the down-ramp), as something similar, except for it not being right for a number of other reasons (including that, though in this case the ramp-end lands on ground, I personally avoid this positioning in case of accidental atom-smashing, so would end it at the bank/chasm-edge itself).
The four corners so defined, for both raising and retracting[1] and maybe even fully-static, these features can be similar to your bridge-edging (that I left on the N-S version, but inconsistently overedited as the lowered 'rope' on the E-W ones!), as far as they extend, and do something simar to the stockpile-edges (but corners only, with a vastly simpler range of shapes therein - I previously mentioned my chequerboard-interleved pair of stockpiles, I think, but you'll never get that complicated with monolithically rectangular bridges). The "raising edge" would be enhanced (or replaced) by the drawbridge/raised-wall graphic. If two or more bridges abutt, this should sufficiently indicate the footprints in levered out (as well as in-place) positions.
Something I often do, BTW, is put four (up-to-)maximally-sized retracting bridges across my 'skylights', with additional lines of floor 'gantries' to split gaps larger than such a simple four-square system can handle. With a 'corner-L', there'd be a floating 'dual crosshair' cross in mid-air. With objectless shadows (as above, if actually shown in mid-air) it might be better understood with a degree of feathering/fade towards the edge.
Right now, though, there's practically no indication[3] of whether/how a raised/retracted bridge is occupying the floorspace (making it something that would block future bridges attempted to be built to overlap what's already potentially there), so maybe there's no real loss if it isn't shown. Except during Placement mode, perhaps, where levered-away bridges and the footprints of those bridges pre-designated but not yet constructed can be given a 'coverage' indicator like a shadow/glow/strobing-diagonal-pattern/whatever just to indicate what cannot be overlapped and to indicate what distance must be covered to ensure a flush-meeting of opposing sides of a double-drawbridge.
[1] I had at one point played with the bridge-deck actually animating to retract into any/all of the required supporting ground, 'louvering' into a compact (not 'raised as wall') fudge attempt go explain where jt's gone. But this might require further in-game info to check which sides are suitably supportive, or remember the valudation granted for it at build-time - plus update this if you built an opposing 'supporting ground' then removed the original. But then, as you know, I sometimes overthink things!
[3] Possibly, and I may be wrong here until I check it, but under vanilla graphics I think the 'empty air' below a bridge always shows a higher-level bridge's footprint, as it does a floor-gantry or any other construction. But that's no use if there's no depth to go and look at (or, in the multi-Z-depth-view coming to Steam-graphics, observed as additional shadowing through the depth-haze) because it's a flat-on-ground atom-smasher or similarly covering not enough depth to resolve.