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Linux ODBC or iODBC or unixODBC work in principle. The idea is not bad but hampered by the fact that you need a compiled version of a database ODBC driver for your platform. As that is not generally something all database providers are eager to do, it makes unixODBC less interesting overall. In cases where you have a native driver like FreeTDS I would usually consider that preferable to trying to get unixODBC to work. unixODBC is an extra complication on top of a driver and the ODBC manager implementation is pretty complex in order to provide version compatibility between both higher and lower version ODBC clients and higher and lower version drivers.. This makes setting up a unixODBC installation more cumbersome. On the upside is of course the advantage to only have to interface to one API and connect to different databases simply by reconfiguring the connection.

The SQL Server protocol has been reverse engineered as it is based on the old Sybase Tabular Data Stream format that Sybase did document at some point in a public document. Microsoft licensed the Sybase code and eventually created the SQL Server product from that. The underlaying network protocol is still very much the original TDS format with some extra features sprinkled into it by Microsoft over the years. This is the MS document from the original Sybase 4.2 version of the TDS documentation. This is a more detailed documentation about the later Microsoft modifications to the protocol. The "official" open source implementation of the TDS protocol is done by the FreeTDS project. Their implementation works also for the more modern 7.x protocol versions used in recent SQL Server versions, but as anything that was reverse engineered (even with proper protocol documentation) there are some obscure features that might not work properly with the FreeTDS implementation. Compiling the FreeTDS sources for a cRIO shouldn't be that complicated. Reimplementing it all in LabVIEW on top of the TCP primitives is theoretically also doable but the encryption features of this protocol will be hard to implement properly in this way.