Is it possible to set a custom list value based on script ID? Or — how does one know the ID to use?

<customvalue scriptid="budget_source_changeorder">
    <value>Change Order</value>
</customvalue>

I think the numeric values are only determined upon deployment in a target account. However, we must use the numeric ids in code. This is a glaring shortcoming of SDF as a development tool IMHO. If someone knows better please enlighten us!

I disagree pretty strongly with that approach. NetSuite is generally slower than I'd like even in the best of cases. This only adds to the performance pain.

I'd cache the results

that's a slight improvement, but if these never change, a permanent cache would be sufficient