Bind dynamic status to map visual

Hello, Power BI forum! Here's an interesting question for everyone.

I have a map visual that shows a number of sites scheduled for constuction projects. Each of these sites has a start date and a completion date. Each site will therefore have a status of either "Not started", "On site" or "Completed".

I would like the colour of the pin to reflect the different statuses, e.g. grey for not started, etc.

So far, it's straightforward, HOWEVER... my customer's requirement is to be able to use a date slicer to select any date on the programme, and have the map update to show which sites are "not started", "on site" and "completed" as per the selected date. This means that the colour of the pin needs to change dynamically based on the date selected using the slicer.

This has got me stumped! I can create a calculated measure which will reference the date field and return the correct status label, but this calculated measure cannot be bound to the 'legend' of the map visual. AFAIK only fields (columns) can be bound to the 'legend', and the contents of a column cannot be changed dynamically by filter context.

So - what's the best way to tackle this? I'm very keen to see what people come up with!