I too have been looking for a Brompton front block backpack that I can take to work with a laptop in it. I wanted a setup that I could carry on my back on days I ride my other bikes to work. My searches came up with nothing. I almost went the route of getting a frame and rigging my Osprey Momentum to be permanently attached to it but then decided not to do it because a rigid frame will be uncomfortable on my back.
Finally decided to get a Brompton folding basket and throw in my backpack in there and singe it to hold it upright and keep it in place. Plus the basket will be useful when I ride with my kid to the park carrying her toys and on grocery runs. Ordered the basket from REI and waiting for it to arrive to try it out.
Interesting to see what others are doing.