Output Descriptor for an Arbitrary Script
Output script descriptors do not currently support any arbitrary script. However once extended by Miniscript, descriptors will be able to support any script logically. Minscript is an extension of descriptors which allows for arbitrary scripts, within certain parameters. While you cannot specify any arbitrary script, you can use Minscript to create an arbitrary script that does what you want. This will not necessarily be the same as an arbitrary script that you come up with, but it will be logically identical.