I'm not sure if you focus on the Websphere liberty version or the clustering feature.

For the Websphere liberty version:

The platform support guide indicates only 7.4 is supported with IBM WebSphere Liberty v17.0.0.3.

I'm guessing IBM WebSphere Liberty v17.0.0.3 is quite recent and didn't even exist when Pega 7.2.2 was released (to be checked).

If so, it is very likely the combination Pega 7.2.2/Liberty(17.0.0.4) has simply never been tested because this liberty version didn't exist at the time.

I would say chances it works fine are high, but unfortunately this needs to be tested.

In term of the clustering feature, I expect this should work as this should be transparent to the Pega application.

The clustering feature provided by Liberty should be transparent for any application (I mean Pega and not Pega  application).