> First off, when I do a view sample data I get a single row with nonsense data in it (ABCEDEF ... etc for strings, and NULL for everything else)
That would be from using the loopback translator. The current incarnation of the loopback translator does not make it useful for reproducing sample data. That was recently discussed here: https://issues.jboss.org/browse/TEIID-2285 but no follow-on issue was yet logged.
> is there no way to test a text data source without deploying
In your case it would seem straight-forward to also deploy a sample data file.
> Clearly, "yyyymmdd" isn't the date format expected. How do I specify the format?
The standard date format is yyyy-MM-dd (the default logic is very restrictive as we need to be precise for pushdown). The workaround is to initially project the column as string and then use a parseDate function in the SELECT clause. You can also log an enhancement if you want to specify patterns as part of the texttable itself.