I don't quite see the issue. If you are using 38mm hose for structural attack, 99% of the time it should be simply run off the pump. To keep this simple and easy, just attach a single 64->38 adapter on a semi-permanent basis to one of your deliveries. If on the odd chance you need to use a line of 38 off a 64, then just attach and adaptor. It's not that hard...
It's not that often that we are using 64mm hose off the deliveries, and its even less often to see an actual 64mm branch in use.
Personally, I've never had an issue with 38mm hose. It puts out fires great, it's manuverable, flexible and generally easy to use. Why do we need to 'fix' a problem that doesnt exist, let alone 'fix' is by using bigger, heavier hose?
Dezza, have you tried rolling your 38 according to FGP 2.23 "Double roll layflat hose on the bight". Makes if far more managable and easier to stow. It can be tough with current appliance designs only allowing for a single roll on the bight, but in some cases (Like our highrise kits) it frees up a heap of space.