Nickel should indeed be smart enough not to switch to USBMS if the device is connected to a plain power charger (as opposed to when the device is connected to a USB host).
I should know, I replicated that distinction myself
here 
.
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And, as @PeterT mentioned, the behavior requested by the OP is available as a hidden Nickel setting, one that can be toggled via NickelMenu (which is a good thing, because I don't remember the name of the config key right now

).