After some research I maybe found an answer by myself for this question, not sure yet. Maybe temporary or at first glance only. 

While I embrace user5482's (Thanks for your fast reply!) approach using hook_menu() in a custom module, what I have considered already, I still have 2 worries. First: there is missing experience on my side and missing documentation on the functions side for its underrated delivery argument (take a look here). Second: Custom modules should only be considered for very rare individual situations, otherwise you fastly run into 10 custom modules on a project which need to be maintained on core updates. If the scenario what you are looking for is more common, the chance is 99%, that there is a module for that.

So, at first, I will try to achieve this with the JQuery_Ajax_Load module, which actually builds on the mentioned hook_menu() and I will report back when I have any news on this.

Internal URL or menu callback to use node/entity content without html/page wrapping
So here is my question: Say we have measured something to be 15,67 mm and the significant error is $\pm 0,01$mm. then we convert the measurement to meter to be 0,01567m would the significant error then be $\pm 0.00001$m?
