The issue is not driving values with other values, the issue is the method. Storing values in custom attributes is as arbitrary as storing them in strings in the user property buffer, or in a text file on disk, etc. I'm not going to add a "Birth From Custom Attributes" operator any more than I would add a "Add Force from values stored in text file" operator, because it's too niche.
A generic "get value from parameter block" operator would be less niche, but then it wouldn't be specific enough for your purposes anyways. Maybe a single property could be read from an arbitrary parameter block, but being able to read arrays of matrices, and then use those values to birth particles...is its own beast that would require handling in non-intuitive ways.
That's why the script operator exists...so that you can basically do whatever handling is required and I don't have to make a million operators for every very-specific-and-unique scenario
By the way it's worth noting: the Script operator is linked up to max's own .NET implementation...so it's conceivable that you could use it to retrieve an IINode of your input object (by name or handle), then read custom attributes from it, then do your birthing. I haven't tested that specifically, but I don't think there's anything preventing it from working in the current implementation.
A generic "get value from parameter block" operator would be less niche, but then it wouldn't be specific enough for your purposes anyways. Maybe a single property could be read from an arbitrary parameter block, but being able to read arrays of matrices, and then use those values to birth particles...is its own beast that would require handling in non-intuitive ways.
That's why the script operator exists...so that you can basically do whatever handling is required and I don't have to make a million operators for every very-specific-and-unique scenario
By the way it's worth noting: the Script operator is linked up to max's own .NET implementation...so it's conceivable that you could use it to retrieve an IINode of your input object (by name or handle), then read custom attributes from it, then do your birthing. I haven't tested that specifically, but I don't think there's anything preventing it from working in the current implementation.