do_add uses ${D} instead of ${ED} or ${EROOT} when ${D} is not set. This results in do_add not using the EPREFIX when used from Portage (since ${D} is set). The attached patch fixes this by always appending the offset-prefix.
Created attachment 210347 [details, diff] eselect-1.2.7-do_add_prefix.patch
In SVN r732. Currently no new eselect release is planned (I did one yesterday ;-) but you may always ask of course.
I'm first going to check if I can find any more bugs, to avoid being at version 1.2.10 in a week from now. Thanks.
Fixed in eselect-1.2.8.