Transparent proxy for HTTPS traffic

I'm entirely sure I understand your layout, but if you just want to give your visitors Internet access (and don't care if they use a proxy at all), why not just add another leg to your edge firewall/router (assuming it can do that) and bridge the wi-fi router onto that leg and route/filter accordingly?

                 Edge firewall/router: drop src 192.168.2.0 dst 192.168.1.0
[ eth0 LAN:192.168.2.1/24 | eth1 Guest:192.168.2.1/24, running DHCP for this network]
    |                         |      
    |                         |      
    |                         |      
 {your LAN}               [wi-fi bridged router (plugged into switch ports, not WAN;DHCP     
                           disabled)]

If you want to filter on domain names, you have two possible ways: you could take the name from the CONNECT method issued by the client if it knows that it has to use a proxy for HTTPS conenctions and filter on that one (Squids supports that BTW). Or, if you really really need to do it transparently, you would need to look into the (encrypted) request headers.

If you want to see encrypted request headers, you need to have a key. If you want to have a key, you need a certificate which is a) trusted by the client to be the "correct" certificate and b) certifies every possible host (wildcard-everything).

So what you would need to do is

1. set up a certificate for your proxy. It depends on your software how to do that - you might use stunnel to terminate the SSL connection at the proxy's side, have some filtering HTTP proxy behind it and re-establish SSL for all outgoing traffic using iptables DNAT targets and stunnel again. There might be "boxed" solutions for MitM SSL proxying as well.
2. install the aforementioned certificate on all of your clients that are going use the proxy

Mostly, if you need transparent proxying it is because you do not want or cannot reconfigure the clients to use the proxy. If this is the case with your request as well, you probably would not have the option to install certificates on clients and mark them as "trusted". So even though there is a technical way for transparent SSL proxying, I suspect not much will be won for you.