So,
I have successfully setup a simple hotspot with a splash page and logon with little issue. It has 2 interfaces and one is pointed to an active internet connection and the other towards a WAP. So I assume the Masquerade rule just uses the 1 IP address I get from upstream and use it for ALL hosts associated with the hotspot.
Here is my dilemma: I have another mikrotik router with 7 ports on it currently in production. I have an entire public class C routed to it and I have several small sub-nets routed out of it. The subnet it uses to connect upstream is private and NOT browsable.
So this router is in a strategic location and has several spare interfaces left. When I setup a hotspot on this router it doesn't work. I assume it is because masquerade only works if the interface has internet access.
I want to know if there is an EASY way to make a small portion of the class C the source of the hotspot internet. IE can I change the masquerade to a sourcenat and then just use a public already routed to the router.

Any help would be greatly appreciated.

Yes. Masquerade is a source NAT, that just happens to determine the preferred source address of an interface when a connection is being established, and uses that as the address to NAT to. 'source-nat' always uses the address hard configured as the 'to-address'. That is the only difference. There is no need for "Internet access", whatever that means. NAT translates addresses in IP headers, that is it. Also, Hotspots don't require NAT to function (well, that's a lie, but they don't require source NAT).

Unfortunately I find the rest of your post a little confusing. If you need specific help maybe draw a network diagram that includes interface names and IP network addressing and then point out what you want to NAT where. That would make it very easy to help you write the required configuration.