Could you possible create some ECO child codes for children of experimental method, that also link to the manual assertion method?
> ECO:0000021 physical interaction x 'ECO:0000218; manual assertion'
With term name = 'Physical interaction evidence used in manual asserion',
( this would then be equivalent to the GO IPI code, as its currently used in GO annotation)
>ECO:0000008 experssion pattern evidence X 'ECO:0000218; manual assertion'
With term name Expression patterhn evidence used in manual assertion
-- would be equivalent to the GO IEP code