Here are some examples of transparent pagers I was testing. Note each has their own issues, but might give you a starting point to get behavior you may want. The transparent effects are only just taking screenshots of the desktop then using that as the pixmap. This has various issues, and a real compositor like picom will give you much better results.
# Transparency
Colorset 25 Plain, fg black, bg DarkSlateGrey, Transparent
Colorset 26 Plain, fg black, bg DarkSlateGrey, Alpha 5, Transparent
Colorset 27 Plain, fg black, bg MidnightBlue, hi LightSkyBlue, sh DeekSkyBlue, Transparent
Colorset 28 Plain, fg black, bg average, bgTint red 85, hi PaleVioletRed, sh DarkRed, Alpha 75, Transparent
Style TP8 ParentalRelativity
DestroyModuleConfig TP8: *
*TP8: MonitorLabels
*TP8: Colorset 25
*TP8: HilightColorset 26
*TP8: WindowColorsets 27 28
*TP8: LabelPixmap true
*TP8: Window3DBorders
*TP8: WindowBorderWidth 3
# RootTransparency
Colorset 29 fg rgb:55/11/55, bg average, \
Tint white 40, bgTint white 40, RootTransparent
#Style TP9 ParentalRelativity
DestroyModuleConfig TP9: *
*TP9: MonitorLabels
*TP9: Colorset * 29
*TP9: HilightColorset * 29
You run them with something like FvwmPager TP9 0 3