Yes, you can configure SWIS to speak HTTP in addition to (or instead of, if you prefer) HTTPS. To do this, you will need to edit C:\Program Files (x86)\SolarWinds\Orion\Information Service\2.0\SolarWinds.InformationService.Service.exe.config and make two changes. (I'll assume you are familiar with XPath. If not just let me know and I'll give a more explicit reference for these edits.)
1. Under /configuration/system.serviceModel/services/service[@name='SolarWinds.InformationService.Core.InformationService']/, add a new endpoint element:
<endpoint address="http://localhost:17781/SolarWinds/InformationService/Orion" contract="SolarWinds.InformationService.Core.IInformationService" binding="basicHttpBinding" bindingConfiguration="HttpBasicInteropBinding" />
2. Under /configuration/system.serviceModel/bindings/basicHttpBinding, add a new binding element:
<transport clientCredentialType="Basic" />
Now you should be able to talk to SWIS using the same SOAP protocol, but with normal unencrypted HTTP over port 17781 rather than HTTPS over port 17778. Authentication is still using regular HTTP Basic authentication. Obviously this exposes credentials to anyone sniffing your traffic, but this is no different than logging into the Orion website without SSL.
Hope that helps. Let me know if you run into problems with it.