i think ur calling ur Default core.txt file a mod ..... and if i am understanding u ,..... then the answer is no
the only way u can use a modded corefile is if the other players u wish to play have the excat same corefile
personalliy i have used this technique a few times ...and i am in the process of builting a ship pack for my fleet to use .... the key is controllling the corefile ... if it is different in any way to any of the other olayers u are connected to ...it wont work
usually gives a nerror like missmatcjhed core .... or something like that
other wise u can use the hardpoint data from and existing ship and add the new ships to the customloadout as a replacement ot and existing ship .... the hardpoints wont always load up ...and ull be the onlyone to see it ... but it will give u the look of the new ship and will NOT affect game play