1. DVD Player to VCR input on Tivo (DVD configured for RGB output)
2. Pioneer Freeview box to AUX input on Tivo (Pioneer box configured for RGB Output)
3. Tivo to Pioneer Plasma (Tivo configured for RGB, connected to RGB SCART)
If your DVD player has component output or your display has a second RGB SCART then you could plug the DVD player in via these connections.
This will allow you to watch the DVD player through your Tivo (by pressing VCR), even if the Tivo is recording a show from the Freeview box connected to the AUX input.
Whatever you do - try to keep everything RGB or component. S-video or composite will needlessly reduce picture quality - which may be more apparent on a larger screen.