Hi,
Yes, iRule could be used.
1) Another perhaps simpler option is to use specific wildcard VS(s) that points to the new ISP for the group of users/email/DNS by specifying their "Source Address" in the VS properties. I believe the "Source Address" feature is available in 11.4.x or 11.5.x onwards.
https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm-concepts-11-5-1/3.html
About source addresses - When configuring a virtual sever, you can specify an IP address or network from which the virtual server will accept traffic. For this setting to function properly, you must specify a value other than 0.0.0.0/0 or ::/0 (that is, any/0, any6/0). To maximize utility of this setting, specify the most specific address prefixes spanning all customer addresses and no others.
2) Website/Web app outbound request/interaction can continue to use existing wildcard VS(s) pointing to the existing two ISPs.