Hey Andrey..
this use-case came to mind immediately when I initially saw the release notes announcing propertySources. but the reasons for not actually using a propertySource include: 1)as a dense, slow and dim-witted "network guy" I've had a hard time clearly and accurately understanding propertySources 2)my level of groovy-scripting competence is zero (see #1) and 3)I thought it would be easier to modify an existing datasource than to start from scratch (so basically I cheated, especially since Johnny Y took all my development notes and did the actual modification).
I think it would be cool & hope that somebody does post this functionality as a propertysource; in the meantime hopefully other Juniper customers might get some value out of this one.