I think I have found at least a partial answer. I found a sample using the rules API, but I don’t know yet whether it is possible to directly edit Biztalk policies through the same library.
The Biztalk SDK Samples folder (C: \ Program Files \ Microsoft BizTalk Server 2009 \ SDK \ Samples \ Business Rules) uses the sample in which the Microsoft.RuleEngine rule is created and then saved in an XML file (using the Business Rule Scheme (BRL) )
I suppose I saw other examples showing how to import this XML file into Biztalk, so at least it looks like a two-step process. I will update if I find otherwise, better or worse.
I am puzzled by why I do not see this task being discussed somewhere on the networks. Either Biztalk is very unpopular, or people just don't use BRE.