If all you want is a bit of control and the ability to switch maps then we already have a solution.
An obd2 cable which allows you to load any file you want.
Peter can correct me if I misunderstood, but I believe he would like it to be more simple than lugging the laptop around with him, and would prefer a simple toggle switch/dial setup.
If your looking into setting up and mapping a standalone yourself then you are potentially entering a world of frustration and high expense. Mapping a standalone ecu to work correctly under various weather conditions is serious hard work and you'll find yourself going through a complete development process.
At the end of it your car will drive not as nice as with a oem ecu and you'll understand why it costs BMW so much money to developed a programme/ecu package for each car.
I think you make mapping seem harder than it is, fuel and timing mapping are the easy part - driveability issues are the hassle.
Mapping Alpha-N is more difficult due to the lack of any actual engine-load measuring. You are basically creating a calibration curve for all feasible conditions - temperature, and barometric pressure - via mapping the engine under said conditions.
However, with MAP and MAFS ecus, the job is considerably easier as all the conditions measured are indicative of what the engine will need, and are not based off of ideal [engine] conditions.
I will agree that tuning tip-in throttle, warm-start, cold-start, and all the other peripherals can be tedious and painstaking; but, with a quality ECU and a quality tuner, it shouldn't be an issue.
Most of the time where people go down the standalone route, they end up getting a professional to map the ecu and that won't be cheap if you want to use someone who knows what they are doing.
Considering the level of some of the builds being discussed, I would not expect them to go the amateur tuning route......
However, as an amatuer tuner (re-tuned my 400hp turbo 2.4l after larger turbo, and new Electromotive Tec3 ECU), it is not that difficult, and my racecar's engine was always reliable, always started, and never had any issues.
I would suggest sticking with the oem ecu and work with someone who is willin to explain and show you the programme. Then get some files made and then all you need is a cable/software package which allows flashing.
I like the OEM ECU, too; but, the number of credible people who thoroughly understand the BMW DME is very very limited, even compared with some of the exclusive standalone units. Not to mention, even the credible BMW tuners have to crack the DME; thus, the knowledge of the DME is learned via experimentation vs. documented knowledge of standalone units.