empheq left tag? or non-emph area

[Dave94705]

I am using the empheq package but for one equation that I want to
emphasize I want to have some text flush on the left side of the same
line but not emphasized. Outside of empheq I can use flalignedat to
accomplish this, but within flalignedat is not allowed.

I want something like:

(Einstein) || E = mc^2 || (eq#)

where the ||'s are not visible but between them, and only between
them, E=mc^2 is boxed, colored, etc.

Any suggestions on a, hopefully easy and direct, way to accomplish
this?

thanks, ds