Need your suggestion about maintenance tunnels management

Scenario :
multiple customer sites to manage, time to time
no port forwarding to managed machines wanted on remote routers
no software like teaviewer, ammyy etc

I was thinking about vpn to each site (gre/ipsec or l2tp/ipsec)
An In-site Mt device should place a tunnel call to main site (this to avoid customer existing firewalls/routers modifications)

But I don’t like an amount of tunnel opened permanently to main site.

What about to set up all tunnels on main site , leave them disabled and enable them only when maintenance to that particular site is needed , then disable it again ??

This could be done via a script recalled for example by a web page

Do all this continuous tunnels tries, affect performance/bandwidth ?
Any suggestion about ?


Thank you