Not strictly true, charge will still take place, just you need to rev high first to energize the field windings, and no you don't want a permanent feed to it!

The way this works is via the charge warning light, power is sent via this to the alternator, so really if your digi dash does not have the capacity to do this you would ideally need to run the old bulb outside the cluster, via a switched live feed, the ground for this is the blue wire, which will be in the connector for your original dash!

I say use the original bulb, as the type of bulb is important as well!

The standard brake warning light is 'tri purpose' and it is negatively switched, in that it is a handbrake warning light, pressure low light and fluid low indicator, so akin to your new setup, can you not take the common live from the old dash and use this, or has this been used and you need another live feed?

Martin