You can get okayish flat pedals that toe-clips will fit on.
I have some VP ones which look a bit like old Shimano DX flats, but have reflectors on the front and back. Take a reflector off, and with some fiddling, you can attach clips to the pedals.
You have to use strapless clips (with no front) – there are plastic ones available in loads of places which work okay.
These look sort of similar to what I have:
http://www.chainreactioncycles.com/Models.aspx?ModelID=16097
http://www.chainreactioncycles.com/Models.aspx?ModelID=5195
On some of them, the body is the wrong way round for it to work, you either need to put something under the clip (like a small block of wood), or swap the left and right pedal bodies.
Although having said that – it takes more time to take off the clips than to swap pedals, and a pair of lightweight road clip & strap pedals is pretty light.
Joe