EASA Regulations may not specifically preclude you from doing what you are planning to. They leave an awful lot of room for interpretation. How you do your maintenance is up to you, as long as you meet the core requirements and that your processes can be audited.
Additional rules for this type of activity would normally covered by a local procedure (such as ...
The answer by pithblot is more or less correct. The 12,500 lb. limit was chosen as an approximate halving of the DC-3 gross takeoff weight.
Consider the following quote:
As noted earlier, when first promulgated, the 12,500 lb. weight limit
could be understood in light of the general use of the DC-3, an
aircraft with a maximum gross takeoff weight of 25,000 ...