You can export custom UMG widgets as a texture output node on Reality Editor.
The widget class must be derived from ZDRealityWidget class.
Click ADD NEW → USER INTERFACE → WIDGET BLUEPRINT on the content browser. You can rename the widget as "Custom Widget".
Double click and open the widget. Simple, add an image into the widget.
Click GRAPH on the right top corner.
Click CLASS SETTINGS then Click Parent Class drop-down under Details panel. Select ZDRealityWidget as parent class.
The parent class ZDRealityWidget is required.
Click EDIT → PROJECT SETTINGS
Scroll down, click on Exports. under the Reality tab.
Click ➕ button on widget class. Expand 0 index. Click drop-down menu on Widget Class. Select widget that you created. Give an export name.
Launch Reality Control and press F7 to launch Reality Setup.
Click PLAY on Reality Editor.
Open Reality Setup. Click CREATE → EXPERIMENTAL → WIDGET → WIDGET
Click Widget node that you created on. Click select your widget from widget menu.
You can use widget as image/video output on Reality Setup.