..I think there is no speicific default body-content for custom tags. It just depends on how you specify the <body-content> element in TLD.
For custom tag(simple tag, custom tag), the <body-content> is required, (mandatory) in TLD.
For tag file, it has a default value of 'body-content' which is 'scriptless'. The tag file 'body-content'is not defined in TLD, rather, it's defined inside the tag file itself using <%@ tag > directive, for example, <%@tag body-content="tagdepent" %>.
For a custom tag, the <body-content> element inside the TLD is mandatory, but a Tag File doesn't have to declare <body-content> if the default (scriptless) is acceptable. As with Simple tags, the Tag File body cannot be scripting either as you mentioned.