Brunetti Cycles (Italy) makes a carbon expander very light (I can't remember the weight offhand, but I had trouble ordering from them direct because of the language problem. However, I have discovered that they do have a US outlet now at firstname.lastname@example.org
Another source is Andrew at email@example.com
He is the new US distributior for TISO in this country and they make an 18 gram carbon expander. I should get one from him next week. He, however, just left for Asia on business so I don't know if you'll get him right now. Cost was under $15 without shipping.
As far as a lightweight top cap goes, contact poshbikes in the UK: firstname.lastname@example.org
They sell a very light (5gm I believe) Storck carbon top cap with a titanium bolt. Fairly inexpensive and the shop was very cooperative. They also have Tune 10 gm. carbon water bottle holders (must be used with Tune slightly concial bottles, but work great)