If I understand Karl correctly, the b/boiler would be connected into the rad circuit, and act as a kind of "radiator in reverse"................ putting heat INTO the water, rather than radiating it OUT of the water.
Just a couple of things............. boiler connections are usually 28mm....... therefore lots of water volume (flow). The rad circuit would have to be mainly at least 22mm, with 15mm only at the take-offs. Without something like that, I can envisage the rad circuit NOT taking the heated water away from the b/boiler at a fast enough rate.
Also, it would be essential that the b/boiler is completely clean internally before sending any of that water through a Combi and contaminating it.
The other thing............ you'd have no b/boiler benefit to the hot water (taps etc). That would stuill be Combi only.