Provided that there aren't hanging wash sales in progress at the end of the year, then the brokerage 1099 records can be corrected to the appropriate net gain/loss via adding back the "wash sale loss disallowed" summary totals.
In this case, TurboTax does actually know the unadjusted basis for all trades -- when TurboTax imports the .txf files from the brokers (or other method), both the cost basis and adjustment due to wash sale are listed. It populates the 8949 forms correctly with both unadjusted cost basis and amount of adjustment due to wash sale with code 'W'.
