Good Q.
This is a Q , that exercised me a fair bit whilst building my V. In my experiments with triple chainring setups using standard measurement BCD like 74mm for a 5 arm spider and 64 mm for a 4 arm these will not fit and clear the clamps as charlesw described in a triple setup. This limits how small a granny gear that can be fitted from commercially available granny gears.
TA specialties make a 28 tooth ring that will fit 92 BCD but as charlesw described in his post the ultegra is a proprietary 94 BCD.
With the eye of faith I think a 28 tooth ring could be made to fit the ultegra spider but I haven't put pen to paper and calculated the dimensions to confirm this.
But even if you could I'm not sure that going down to a 28 would make a practical difference on the road vs a 30.
I 'd say that for your legs to feel the difference a step down to 25 would be my starting point.
Something custom made opens all sorts of possibilities . You could for example instead have a triple but on a longer crankshaft so the inner ring sits just slightly further out from the BB clamps , but then the FD may not have the reach for the outer ring. Or perhaps a double setup could be made where the granny gear sits in the chain line position of the middle ring.
In practice it might be easier to find / make a big cassette ring at the back. SRAM make a 36 tooth cassette ring but with a long cage dérailleur you could go bigger custom made. If memory serves me correctly you tube has videos of cassettes ,chainrings , derailleurs made by enthusiasts at home........
I was genuinely surprised and very pleased during my recent series of brevets that in spite of my earlier worries! a 30 granny ring served me really well, For the whole trip. Normally on my RWD recumbent I use a 23 granny .and use it........ frequently.
Thats not to say I wouldn't fit something something smaller than a 30 on the V. if I could.
Where the terrain is consistently above an 8% grade or at night when you've been on the bike for more than 24 hrs already it would be nice to just spin my way to the top when you're facing a big hill whose summit you can't see because its so dark.Thing is when I get to that point I get off the bike and walk .
There is also another possibility that might be of interest, adopted by many an enthusiast who do heavy loaded touring to all parts of the world........ Rohloff hub .........