Well first thing you should know is that Tuxedo 8.1 is past its end of life date, so only sustaining support is available for that version, meaning no new bug fixes.
Probably the simplest way to handle this is to use a later version of Tuxedo such as Tuxedo 12.1.3 as a gateway between the .NET application and the Tuxedo 8.1 domain. Essentially you install the later version of Tuxedo, configure the domain gateway on it and the Tuxedo 8.1 system, and import the Tuxedo 8.1 services into the later Tuxedo domain. The .NET client then connects to the later Tuxedo version and it relays the requests to the Tuxedo 8.1 version. Another alternative would be to use SALT which provides a Web service interface to Tuxedo applications, although you would then need to use the .NET framework 4 or later.
Oracle Tuxedo Chief Architect