It is possible to combine copper foil in a leaded glass panel.
The copper foiled piece should be soldered before inserting it into the lead came. In this way the soldered together pieces become very like another piece of glass.
There are some special considerations, of course.
The copper foiled piece should be designed as though it were a single piece of glass and so can be accommodated into the surrounding pieces of glass. Copper foiled piece should not have severe undercuts which would make it difficult to insert into the surrounding glass. It may be necessary to incorporate a piece of the surrounding colour to make it fit into the panel.
The copper foiled piece should be finished with all the beads on both sides. If one side is left flat, it will collect water if on the outside, and catch on any cleaning processes whichever side it is on. However, the piece should be tinned only on the outer edges. This will ensure that the copper foiled piece will slip into the came.
The image below illustrates a copper foiled piece incorporated into a leaded panel.
This section of the panel shows the accommodation of the main leaded panel with the copper foiled piece with a line from a petal to a leaf. Otherwise, it was fitted as one piece.