I think there are two somewhat separate issues here. First, completely replacing the Specify attachment server with an independent digital asset management solution. And second, capabilities within Specify for linking or attaching to resources that are altogether external to Specify.
The second issue is simpler since the sorts of external resources one might want to link to are so heterogeneous that you really can’t do much more than store a URL. Currently, Specify does have a WebLink form element that can be associated with fields in the database. It allows basic linking out to resources. It can also build URLs based on other types of field data. For example, you could store an id number and configure the button to substitute that value into a URL. It seems like this is pretty close to what you are suggesting. Are there specific capabilities that we could add there?