line命令での(擬似)作成は以前からやっていたので参考までにどうぞ。
#module
#const GROUPBOX_LEFT_PADDING 7
#const GROUPBOX_RIGHT_PADDING 7
#const GROUPBOX_LEFT_SPACE 3
#const GROUPBOX_RIGHT_SPACE 3
#const GROUPBOX_LEFT_OFFSET (GROUPBOX_LEFT_PADDING + GROUPBOX_LEFT_SPACE)
#const GROUPBOX_RIGHT_OFFSET (GROUPBOX_RIGHT_PADDING + GROUPBOX_RIGHT_SPACE)
#const GROUPBOX_BOTH_OFFSET (GROUPBOX_LEFT_OFFSET + GROUPBOX_RIGHT_OFFSET)
#deffunc groupbox str _caption, int _width, int _height
r = ginfo_r
g = ginfo_g
b = ginfo_b
x = ginfo_cx
y = ginfo_cy
;フォントの高さを取得
mref bmscr, 67
font_h = bmscr(32)
bmscr = 0
;キャプションを描画
syscolor 18
pos x+GROUPBOX_LEFT_OFFSET, y-font_h/2
mes _caption
capt_w = ginfo_mesx
capt_h = ginfo_mesy
width_ = limit(_width, capt_w+GROUPBOX_BOTH_OFFSET, _width)
;枠の影を描画
syscolor 22
pos x+GROUPBOX_LEFT_OFFSET+capt_w+GROUPBOX_RIGHT_SPACE, y+1
line x+width_, y+1
line x+width_, y+_height
line x+1, y+_height
line x+1, y+1
line x+GROUPBOX_LEFT_PADDING, y+1
;枠を描画
syscolor 16
pos x+GROUPBOX_LEFT_OFFSET+capt_w+GROUPBOX_RIGHT_SPACE, y
line x+width_-1, y
line x+width_-1, y+_height-1
line x, y+_height-1
line x, y
line x+GROUPBOX_LEFT_PADDING, y
;ginfo_r, ginfo_g, ginfo_bを元の値に戻す
;ginfo_cx, ginfo_cyをグループボックスの下の位置にセット
color r, g, b
pos x, y+_height
return
#global
syscolor 15 : boxf
pos 5,5 : groupbox "hoge", 200, 100