The question which started my search is that our
    requirements ask for a transparent end-to-end
    asynchronous data service. Would transporting
    a break condition be included? and transporting
    strange things like fractional data bits?

The only way to find out is to ask your customer what they mean by
transparent.  It can mean anything from ascii only to 8 bits plus
transmitted parity plus transmitted signal state changes (RTS/CTS DTR/CD
for fax modem manipulation, for instance), plus relative timing between
bytes (Some host software can distinguish between function keys and user
typein of the same sequence based on timing.  A real bitch.)  Even
"break transport" needs a tighter definition - I've used equipment where
there is a distinction between "short" and "long" breaks (but I don't
think I've seen a requirement for that sort of transparency in quite
some time...)

BillW