I can only speak as a Cantabrian who follows the Mainland leagues.
Having the top club(s) from the Mainland League compete at the end of the winter season in a national club competition would be a huge backwards step.
The likes of Cashmere Technical are playing in what to be honest is quite a mediocre league and would only be able to field a fairly weak side in the national play-offs.
Cashmere Technical draws "crowds" of about 80 people.
Canterbury United, even when results have been poor, have had amongst the biggest crowds in the summer national league, often about 800. Better than the more successful sides Auckland City, Team Wellington or Waitakere.
The Southern League is even weaker (Cashmere Tech thrashed Southern champs Green Island 4-1 in Dunedin recently in the South Island championship final, starting some of their best players on the bench)
Successful Christchurch sides in the national league have generally always been a combined effort with several clubs pooling their resources to create a stronger side.
(Except for a brief couple of seasons in the late 1990's when Mainland club champs Woolston qualified for the national league and were an abject failure)
Christchurch United was formed in 1970 specifically to compete in the new national league by the combined efforts of clubs Christchurch City, Shamrock, Western and Christchurch Technical.
The current Canterbury United grew as a franchise organically after Woolston AFC's failure, when Woolston first merged with Christchurch Technical to form Christchurch City to compete more strongly in the national league and then Mainland Football took over and created a Mainland representative franchise.
The "Super Club" 1993-95 era of regional league champions playing off was an abject failure which failed to capture the interest of supporters.
So it's been tried before and failed.