You can import into a subset. You should create a module with the full list as a dimension (you should have a List Management module actually), and two boolean line items, one that can be a formula the other a user choice.
Create a view in the module only containing items that have been set as true. Go to the list > import > from this view > import the boolean into the subset
The end user will be adding forecast assets to the list via an action button, how do I incorporate the import within a process that runs in the background to ensure that the calculating modules have the assets available to them to perform relevant HP, IFRS16 calc?
You mention that the user runs an action to update the list. Assuming you have a boolean line item to drive your subset, would it help if you use a process instead and add your current action and the subset import action(from your boolean line item) to the process and let the user run that?
An additional complication is due to forecast versions. An asset could be assumed to be purchased on HP in one version and cash in another forecast version.
A boolean in a system module for assets will pick up the selection made in the forecast version that is current. The dynamic list subset does not work across forecast versions.
While the calculation modules need the assets to be available to them based on the selection made in the relevant forecast version for an overall comparative view. What if X in capex is funded through a mix of HP and Cash vs Y.