I have added your question to a previous discussion of this topic.

This thread describes a strategy for transferring USA Membership Rewards points to British Airways, and then transferring to a related program called Avois, which can be used to book Eurostar passage. Background: the loyalty currency named "Avois" is used by three programs: British Airways, Iberia Airlines and the freestanding Avois program. To join the non-airline Avois program a UK address is required, but the thread suggests it can be any address. Read about the program here: http://www.avios.com/gb/en_gb/

There is also this recent post which clarifies that Eurostar redemptions through Avois do not have a fixed 15,000 point level as mentioned in the thread.