Whilst the new Lego base layer is not officially supported for non-plus models of the Raspberry Pi B (“layers are only suitable for the Model B+ Pibow. They will not work with the model B, A or A+”), it would seem that provided you use one of the newer Model B Pibows, such as the Coupé, the new modification base layers will fit.
The key determinant seems to be the plastic bolt hole placement. If your Pibow has these right up against the corners, the modification layers do seem to fit. But if you’re got an old Pibow where two of the bolt holes are around a centimetre away from the GPIO edge, then they won’t fit.
Here’s a Pibow Coupé B with an original, day-one-release Model B. Note the black audio connector, and note that I had to desolder the JTAG pins to get it to fit (I say “desolder” but it was 90% tin snips to be honest).