Holacracy Community of Practice Archive, 2015-2019 Community Holacracy Web Site
As I see it, the action of the Lead Link role substituting for the absent role is no different, mechanically, than the Lead Link accepting the request from outside the circle. As Martina states, the lead link accepts the project for the role but this does not mean the Lead Link orders the role to execute the project. There are some things that the absent role holder is obligated to do under the constitution 

4.1.2 Duty of Processing


You have a duty to promptly process messages and requests from your fellow Circle Members, as follows:


(a) Requests for Processing: Other Circle Members may ask you to process any Accountability or Project on a Role you fill in the Circle. If you have no Next-Actions tracked for it, you must identify and capture a Next-Action if there are any reasonable ones you could take. If there are not, you must instead share what you’re waiting on. That must be either a Next-Action or Project tracked by another Role, or a specific event or condition that must happen before you can take further Next-Actions.

(b) Requests for Projects & Next-Actions: Other Circle Members may ask you to take on a specific Next-Action or Project in one of your Roles in the Circle. If you deem it a reasonable Next-Action or Project to take on, you must accept it and track it. If not, then you must either explain your reasoning, or capture and communicate a different Next-Action or Project that you believe will meet the requester’s objective.
So, i feel, no shadow power issue arises. Under the constitution you are obligated to consider project requests but how you wish to conduct your attention and resources is within your authority.

1.2.5 Directing Attention & Resources


Whenever you have time available to act in your Role, you are responsible for considering the potential Next-Actions you could efficiently and effectively do at that point in time, and executing whichever you believe would add the most value to the Organization from among that subset.
I hope this is helpful.