You'll configure and allow hairpin NAT on your SRX (http://kb.juniper.net/InfoCenter/index?page=content&id=KB24639&actp=search), make sure to allow the traffic from the VPN zone to the other VPN zones for services that are needed. Depending on your setup you'll either add the customer network list to the protected networks for split tunneling on the clients, if it's not split you'll don't have to add anything client side.
On the site to site VPNs you and your customers will have to add your VPN subnet to their cryptomaps so the client VPN traffic will be encrypted and forwarded to the remote sites.
If you are using a pulse appliance for the VPN the configuration could be more simple if you just NAT it out of the pulse box to an already trusted network, that way the traffic appears to the SRX as already coming from the inside and the client sites will already know of the network that it's coming from so no further configuration on their end would be needed.