Please see the topics Layers in R80 and How do I create an Access Policy for Pre-R80 GWs? for the list of the supported features.
R80 Management has the support for inline layers, however like you said, when using them for a pre-R80.10 GW, install policy will fail.
Setting an inline layer is done by clicking a rule's action and selecting the "Inline Layer" option. You can either select an existing layer (if it's marked as shared) or create a new one.
The way that inline layers work is the following: When the connection matches a parent rule that its action is an inline layer, the inline layer rules get evaluated.
Every inline layer (and also every layer) has an implicit cleanup rule that is either "any any accept" or "any any drop" set in its properties under "advanced". This means that once you go inside an inline layer, you cannot go outside back to the main layer, therefore rules in the inline layer cannot block rules that reside below the parent rule that holds them. Giving an admin the permission to only edit an inline layer will not affect the main layer that holds it.
To see the list of all layers, open the Manage Layers view from this location:
