Unfortunately it's not that easy. For starters, RJ45 is compatible with RJ11 because it has 8 wires, to RJ11's 4. For analogue and (believe it or not, still in use) ISDN intercompatibility when installing RJ45 jacks they just wire up the same along lines 4 and 5. 3 and 6 should cover a dual line from a single RJ11 I think, but that still doesn't cover the 8 wires that RJ45 needs. That "adapter" you have found works simply by leaving out the four wires that RJ11 doesn't have on each end.
Then you have the hardware. Well, the BBA is made to work with RJ45, for all 8 wires. What's going to happen when it doesn't have them? Also, the Dreamcast modem needs a dialtone to get going. Plug it into an ethernet network and it won't have one.
Google around and you'll find info on how to set up your computer to handle a dial-in from your Dreamcast and route an IP from your cable/DSL to it.
