Is there any way to have a custom tag that supports a variable number of unknown attributes?

No, you cannot place a dynamic number of attributes in the custom tag as the JSP engine expects to validate a static number of attributes. An alternate approach to this is use a series of inner tags within your outer tag that set these dynamic settings, e.g.,

<custom:form>
  <custom:attr name="someUnknownAttr" value="1">
  <custom:attr name="someSpecialAttr" value="2">
</custom:form>