When using Text mode, the functions works correctly.
My guess is that the first character of a UNC path which is a backslash is considered as an escape sequence. But it seems you can't escape the first two backslashes when using raster2pgsql (tried \\\\, /\/\, doublequoted path and every combination of these three).
My configuration : Windows 7 Postgres 9.3.5 Postgis 2.1.3 Database using WIN1252 Encoding and French_France.1252 collate. Client encoding : UTF-8