I would give more thought to improving the bar postion or adding gears on the Brompton. I dont tour and I recognise that you have alot of experience that have formed your priorities. I would look at fitting bullbars about 45 cm or wider as they fold flat on a brompton either side of the front wheel. With a modified 8 speed afline you can lace that to a road race brifter callled a versa. Just for information if you dont know this is available.
You can fit a carbon crack but you need to get the right width bottom bracket as it wont fold if its the wrong width. Cant remember to size at moment.
Did you consider other set ups like 8 speed or 5 speed or a front mech. I choose proper gear spread and increments over weight saving anyday.
Would like to see photos of your set ups when done. Thank you