CAD tip # 2063:

(vl-load-com)
(defun openRO (fn)
 (vla-activate (vla-open (vla-get-documents (vlax-get-acad-object)) fn :VLAX-TRUE))
)

(openRO "C:\\documents\\mydrawing.dwg")

(openRO (getfiled "Open R/O" "" "DWG" 0))

You can then also redefine the OPEN command to open all DWG files as read-only:

_UNDEFINE _OPEN
(defun C:OPEN()(openRO (getfiled "Open R/O" "" "DWG" 0))(princ))

(defun C:OPENEDIT()(command "._OPEN)(princ))