Hi Karen,
I'd say the detection limit would be calculated the same way, which as you say will make it much more difficult to get sufficient detection limits for trace element mapping.
You'd also have to perform an accurate background correction, so you either have to collect at least one separate off-peak map (and use a slope factor) which basically doubles your acquisition time, or, if the major element composition of the phases you map is very uniform throughout, you might be able to determine an accurate background intensity using point acquisitions.
@John - I've never checked - is there actually a direct way in CalcImage to use externally determined "fixed" background intensities?
To achieve the detection limits required you'd have to use long counting times, high beam currents (hundreds of nA) and/or accelerating voltages. The drawback of the latter is that correction factors get very large. You might also run into problems with sample modification by the beam and/or charging. Some people use uA currents for example to map traces in steel and for that you also need to protect your BSE detector.
Combining spectrometers is certainly possible.
Multiple frames as on an SEM could reduce beam damage and charging, but if you do stage scanning to avoid WDS defocussing you'd lose additional time when the stage drives back to the beginning of the next line. Depending on your microprobe there might also be some shift between frames or even lines (stage and/or beam related), so it might only work well enough at lower magnifications. Maybe John can also chime in, but as far as I can see the only way to collect multiple frames of the same map in Probe Image is to add them as multiple acquisition samples and combine them afterwards?
Cheers,
Karsten