Is there a way to perform a statement such as

update myRecords SET type=99 WHERE username='Fred'

without calling with a raw TSQL command? I'm using the LinqMetadata approach for most of my solutions, but what is a suitable way to at least build an appropriate WHERE clause to pass to the execution?

Updating entities directly in the database is done with the UpdateEntitiesDirectly API on the dataaccessadapter class, see https://www.llblgen.com/Documentation/5.9/LLBLGen%20Pro%20RTF/Using%20the%20generated%20code/Adapter/gencode_usingentityclasses_modifying.htm#using-updateentitiesdirectly

Not linq, but you can use the QuerySpec extension methods to formulate the predicate, so the where clause in your example is then constructed as: MyRecordFields.Username.Equal("Fred")