async fast-path calls?

hi,

is there any easy way to issue large object operations like lo_write,
or more generally fast-path calls, asynchronously?  (async in the sense
of PQsendQuery so that i can poll to wait a result)

background: i want to use multiple connections to the server in a single
application for performance reasons.  the application is single-threaded.
(multi threading is not possible because i need to use a
pthread-incompatible library.)
using synchronous api when another asynchronous command is in-progress is
deadlock-prone as i can't receive the result of asynchronous one until
the synchronous one finishes.

YAMAMOTO Takashi