...Interesting, and you find it okay to straddle the top tube without having to sit on it? Sounds like I could go up to at least a 530mm or even 540mm seat tube, raising the headtube below the top tube by a similar amount - there'd still be reasonable amount of seat post showing, and am pretty sure the stem would be low enough (I don't think I would want the bars as far beneath the saddle as in the photo of the Cooper), though I guess whether I'd be able to straddle the top tube comfortably would depend a little on the bottom bracket height (which I couldn't tell when in the jig, as that was high off the ground).
The seat stays wouldn't be in the same position, though, which I guess could change the feel of the bike?
the blue bike in the photo above would still have a head tube longer than the exposed seat pin if it were built ~1" bigger, so would fail my (admittedly highly subjective and maybe retro) rule of thumb. More practically speaking with frame sizing like that only a tall stem would give you a handlebar anywhere near level with the handlebars.
Years ago (before the use of a-head on road bikes) I realised that the exposed part of the quill stem was pretty flexy so I had a road bike built with an extended steerer and used a spacer between the threaded and adjusting races of the headset. This meant that there was less flex. [It also meant that I could have raised the stem to a level where the wedge was close to - and therefore stressing more- the threaded part of the steerer, which would have been a bad thing; in reality I accepted a smaller range of height adjustment in return for a bit of added stiffness.] This scheme had the advantage that a few minute's work could transform the steerer to a 'standard one', whereas shortening an extended head lug is a bit more involved.
Re straddling the top tube; because of how the saddle height is usually set, on a road bike the distance from the pedals to the ground can be about the same as the distance of the saddle top to the top of the top tube, and you can still straddle the top tube OK. This minimum is thus about 4" or so, in contrast to the ~8" or so that might be the current trend when you are fitted on a frame of about that size.
Fashions vary; a long time ago (in the 1950s) the fashion in the UK appears to have been to use as large a frame as possible, more or less. If you had more than ~3" of seat pin showing you were in danger of running out of it (seat pins were built short back then) and might be advised to get a frame 1" bigger. Fit-wise this made little difference since frames were often built with hardly any variation in top tube length. I also wonder if in fact this was entirely pragmatic, bearing in mind that larger frames ride so much better (more comfortably) even if they are built in fairly heavy gauge tubes.
In the 1980s IIRC campag built their aero-styled seat pins in two different styles; one with a 'long' aero section (for racing bikes) and one with a short aero section (for touring bikes, very small frames or for those adhering to a fit using a larger frame size). The long version allowed the saddle (varying with rail height etc) to be between ~6" and ~8" above the top tube and the short version was about 2" lower than that.
So (bearing in mind you don't want to use very low set handlebars) I think you might be OK with a drop from the saddle to the top tube (of a standard frameset i.e. one that does not have extended lugs) of about 6"-7" . Very much less than this and you are going to run into straddling issues and very much more than this and you will not be able to use a standard quill stem, probably. I dunno how that compares with the frame design as you have it at present.
cheers