I think I will wait for a eM Client Portable version
You can suggest a Portable version install for others to vote on via the Sleekplan page below.
However a USB-Pendrive install would be “a very limited / very small user base” who would even use that and not likely even worth programming for.
There is though a Chromebook version planned in the Android app you could use as an alternative portable use device on a eg: small cheap 10" size.