The '.' character is allowed in XML names, so it does not need to be escaped. See the XML specification for details.
But for future reference, the rules for escaping names of nodes, properties, node types, etc., in XPath are described in Section 22.214.171.124 of the JCR 1.0 specification:
The names of elements and attributes (corresponding to nodes and properties, respectively) within an XPath statement must correspond to the form in which they (notionally) appear in the document view. This means that spaces (and any other non-XML characters) within names must be encoded according to the rules described in 6.4.3 Escaping of Names.
The values of attributes, on the other hand, are not escaped in the XPath usage of document view (as opposed to the export usage, see 126.96.36.199 Multi-value Properties, 6.4.4 Escaping of Values and 188.8.131.52 Searching Multi-value Properties). ...
Section 6.4.3 basically talks about how to escape any characters that are not allowed in XML names.
Using XPath was defined in JCR 1.0, but deprecated in JCR 2.0, so the 2.0 specification does not talk in particular about escaping names in XPath. However, JCR 2.0's Section 7.4 does correspond to JCR 1.0's Section 6.4.3.