SalesForce.com(SFDC) to Corticon integration is done using RESTful API/JSON payload. 
In order to use the serializer in SFDC, the attribute names cannot start with an underscore "_".   
Given the Corticon REST / JSON API includes a mandatory attribute __metadata, this prevents out of the box SFDC web service integration in APEX.
The required attribute "__metadata" goes against SFDC allowed naming conventions i.e. having an attribute start with an "_". 

Is it possible to drop the "__" or change the mandatory attribute to something like "object_metadata".
Given that this is a required attribute, are there any other naming options available in Corticon ?

From SFDC' documentation : The name can only contain characters, letters, and the underscore (_) character, must start with a letter, and cannot end with an underscore or contain two consecutive underscore characters. This field is inherited from the Metadata component.

Not only starting with an underscore, but also having 2 consecutive underscores causes issue when attempting to invoke Progress Corticon RESTFul API using JSON.

The __metadata parameter is a hard coded value in the back end JSON implementation. 
As indicated in Corticon documentation says, “__metadata” is a String Attribute name with a value of a JSONObject and is required to describe the type of the Entity. (at minimum)
Inside this JSON Object, additional name-value pairs are added to describe that Entity

To change this _metadata parameter to some other value/some other option or parameterize it to be application agnostic is an enhancement request.

You may request this enhancement through the Progress Community via an Ideas submission.Your feedback is valuable and Idea submissions are monitored by our Product Management team. Enhancement requests are reviewed during the planning phase of each new product release and a list of the enhancements chosen for implementation can be found in the[Readme.txt file/Release Notes document] that accompanies each release. Once you have submitted your Idea the Progress Software Community will have the opportunity to comment on and vote for your Idea.

For detailed information on how to submit an Idea please visit our knowledgebase at :  How to submit an idea for a Progress product enhancement
You can refer to your Idea at any time for updates on the status of your request.

Reference to documentation :
Corticon Server: Integration & Deployment Guide, Chapter Request and response examples , Section JSON/RESTful request and response messages : About creating a JSON request message for a Decision Service

Idea filed in the community:
https://community.progress.com/community_groups/products_enhancements/i/business_rules_management_system_corticon/salesforcecom_apex_friendly_variables_json_attributes_for_restful_api