Indeed DEFPARAM flatafter = time does not allow anything other than a TIME.

To be able to exit on selected days, at a given time you can use OpenDayOfWeek and OpenTime:

If OpenDayOfWeek = 3 or OpenDayOfWeek = 5 Then
   If OpenTime = 190000 Then
      If OnMarket Then
         Sell at Market
         Exitshort at Market
      Endif
   Endif
Endif

This will close any open trade at 190000 on Wednesday and on Friday.

With pending (Limit or Stop) if already triggered you can use the above snippet, while if not triggered they are automatically CANCELLED the next time your code is executed, so you just need to apply the above (modified) snippet not to enter at market after a given time:

If OpenDayOfWeek = 3 or OpenDayOfWeek = 5 Then
   If OpenTime < 190000 Then
       BUY       1 contract AT MyLongEntryPrice  LIMIT  //or STOP
       SELLSHORT 1 contract AT MyShortEntryPrice LIMIT  //or STOP
   Else
       Sell      at Market
       Exitshort at Market
   Endif
Else
   BUY       1 contract AT MyLongEntryPrice  LIMIT  //or STOP
   SELLSHORT 1 contract AT MyShortEntryPrice LIMIT  //or STOP
Endif

in this case you place a pending order, on Wed. or Fri., only before 190000 (while you make no check the other days). You will also close ALL open orders at 190000 on those two days.