Not seen one of those, but I have once managed to change valve springs without taking the head off. It worked for me, but then I was feeling lucky! To explain, here's the "prequel" as they say in the movies!
While thrashing down the M1 at speeds I should not, I broke a piston ring in my Herald 1200 engine. Having just left college, I could not afford a rebore. So I bought a cheap (Â£35) Dolomite 1300 engine from the local paper as I needed an engine quickly so I could get to my first job, and save money for a rebore etc.
Although it never actually let me down, that 1296 engine was a dog, but at least it kept me mobile. About 2 miles from home on the way back from work, it suddenly dropped onto 3 cylinders at about 60mph, and made a rather strange tapping noise. I pulled over and stopped the engine. No holes in the block, all spark plug leads on, so I took the rocker cover off and found a broken valve spring on no 4 inlet. FORTUNATELY the valve had not fallen into the engine, which had stopped with no 4 at top dead centre! It had merrily bounced up and down in its guide at nearly 4000rpm while I pulled off the road. See what I mean about being lucky?!!!!!
I took the plug lead off, wired the valve to the rocker shaft, took the pushrod out and drove home on 3 cylinders and got on the phone to order a new set of springs.
Having just put the engine together, I did not want to have to take the head off again, so devised the following cunning plan.

1. Ensure the cylinder to be worked on it at TDC and the engine will not turn (eg leave car in gear).
2. Take a suitable screwdriver or other round bar, put a rubber piece on the shaft to protect the spark plug hole threads, and insert so that the blade is under the relevant valve. I used the rubber bit that sits inside a spark plug socket as ideal!
3. Using a ball-joint splitter (one of those that is a forked wedge) under the rocker shaft, lever down the valve spring enough to remove the collets. This could be done with one bar, but it's easier if you can compress the spring symetrically.
4. Release spring tension and remove the spring cap and spring.
5. In the best words of Hayne's manuals, assembly is the reverse of the above. Sadly this is where three hands can help. as it may be necessary to hold the valve fully up with the screwdriver while compressing the new spring, and then fiddle the collets back into the cap. However I'm pretty sure I managed to do this on my own somehow.
It all sounds terribly amateurish and involved, but once I'd done the first spring, I reckon I did the rest in about the same time it has taken to write this.
Not sure if this is do-able with double springs, but I did this quite easily with Spitfire (ie slightly stiffer) spec single springs.

Oh, and one final precaution that should have been step 0!!! My luck almost ran out in that I dropped a spring collet into the oil drain from the head. It was still in the sump when I finally dismantled and scrapped the engine! Had to find a spare collet too, so I suggest you put a cloth over the pushrod and oil drain holes.
It might also be worth buying or finding one or two spare collets. If your hand should slip at the wrong moment, the spring is likely to send the cap and collets every which way. While the cap is large enough to see, you can guarantee sods law will lose the collets for you.

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Valve Spring compressor.

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