I'm sure there are other ways to do this as well, but you could get this done by using a FeatureWriter and a SystemCaller. If you write a .ffs file with the feature writer with a dynamic schema definition to the following path:

C:\Users\<usename>\AppData\Local\Temp\FFS_workaround@DateTimeFormat(@DateTimeNow(),%H%M).ffs

Then connect the Summary port to a SystemCaller with the following Command Line Argument:

"<PathTo2021Install>\fmedatainspector.exe" @Value(_dataset)

This will automatically open the data inspector with the temp dataset pre-configured as the file to open. Note, if you go with this, you'll likely want to use the timestamp to prevent any file locks.  

You could try editing this key in the Registry:

HKEY_LOCAL_MACHINE\SOFTWARE\FMEDataInspector2021.2.21784.win64\Capabilities\shell\Open\command

...where the number is obviously 2020.2.uyour build num].win64

Not that I'm going to recommend that, but if I were the sort of person to make random registry edits, that's where I'd start. I'm not sure what it would do to the Visual Preview tool inside WB either. 

HKEY_LOCAL_MACHINE\SOFTWARE\FMEDataInspector2021.2.21784.win64\Capabilities\shell\Open\command

Thanks Mark, but alas, it doesn't work.

I changed the open/command registry setting for the 2020.2 Data Inspector to target the 2021.2 DI, and created a simple workspace in 2020.2 Workbench, and clicked the green looking glass.

And the 2020.2 Data Inspector came up :-S

It doesn't seem that Workbench is using this registry entry to start the Data Inspector.

C:\Users\<usename>\AppData\Local\Temp\FFS_workaround@DateTimeFormat(@DateTimeNow(),%H%M).ffs

"<PathTo2021Install>\fmedatainspector.exe" @Value(_dataset)

This is unfortunately not what I want. It's a general issue, that I want to work always, not one associated with specific workspaces.

HKEY_LOCAL_MACHINE\SOFTWARE\FMEDataInspector2021.2.21784.win64\Capabilities\shell\Open\command

Would it be possible to find out (with the relevant developers), exactly how Workbench is starting Data Inspector ?

And whether it's possible to tap into this to start another version of DI ?

I know for a fact, that Workbench checks to see if DI is running when starting a translation. Because if DI is closed before the translation is done, no DI output is generated. If DI is closed before running the translation, DI will be started automatically.

I don't use Visual Preview much, so that's not an issue.

Using that FeatureWriter + System caller trick could be saved as a custom transformer (Call it InspectorWorkaround or something) so it can be reused in any workspace. It would be just like the old days where you connect an Inspector to any ports that you want to visualize in the DI.

Luckily for me, a replacement service was created by the same entity, that created the original but now defunc'd services. And these work in 2020.2, so I'm back on track. Unfortunately no real answer worked, replacing all references to DI 2020.2 with DI2021.2, so I'll leave it with this.