Both SYM and XCSoar claim to have this feature but it will not get any information from the computer. (It will "sort" of work if no computer is connected )

That is not quite correct actually. XCSoar will always take the data with the highest precision available. If you connect it to a vario it will use those values and if you only have a GPS connected it will try to calculate the missing values from it.

It is actually the same algorithm that makes it possible to connect multiple devices and merge their data to display it on the screen. (e.g. Flarm and Vario)