I got this to work, but it's not what I'm looking for:
(cond
((= (substr (vl-string-right-trim ".dwg" (getvar 'dwgname)) 18 1) "L")
(setq *Xref* (strcat (getvar "DwgPrefix")(substr (vl-string-right-trim ".dwg" (getvar 'dwgname)) 1 16) "DRH-BasePlan-L.dwg")))
((= (substr (vl-string-right-trim ".dwg" (getvar 'dwgname)) 18 1) "R")
(setq *Xref* (strcat (getvar "DwgPrefix")(substr (vl-string-right-trim ".dwg" (getvar 'dwgname)) 1 16) "DRH-BasePlan-R.dwg")))
)