Hi Steve,
What you tried should work BUT!!!
The possible reason for the lack of success here is, by putting the Barlow before the diagonal, that the Barlow lens itself is further into the light cone than where it was designed to be and as a result the focuser travel (out) is not enough to focus - an extension tube to move the Barlow further out will help.
Please note that magnification may be increased by moving the eyepiece further and further from the Barlow this gain comes at a price ... vignetting.
Barlows work by diverging the light beam and as they refract the light there is dispersion so if there is no chromatic aberration expect a little CA if you use an "ordinary" Barlow lens.
Rgds,
Jerry.
