Legacy will will put both block and inline customs tags inside a <paragraph> element in ezxmltext. Block custom tags will however be placed in temporary namespace

    <section>
        <paragraph
            xmlns:tmp="http://ez.no/namespaces/ezpublish3/temporary/">
            <custom name="factbox" custom:title="factbox" custom:align="right">
                <paragraph>block - custom tag</paragraph>
            </custom>
        </paragraph>
        <paragraph>
            <custom name="strike">inline - custom tag</custom>
        </paragraph>
    </section>

However, in a old legacy db I find custom tags just below <section>. I am not able to reproduce this on 5.4 in neither OE or simplified xml, so I assume this is some old behavior:

      <section>
        <custom name="linebreak"/>
        <header>Advisers:
          <custom name="linebreak"/>Foobar Corp.
        </header>
      </section>

Custom tags in headers needs to be considered inline ( and they are in conversion script).
IMO it sounds natural to treat custom tags just below section as block, but that is not possible in this case (the same custom tag name is used in both places).

I see 3 possible conversion solution

1. <section><custom/></section> is transformed to <eztemplate/>
2. <section><custom></section> is transformed to <para><eztemplateinline></para>
3. convers script gets a command option where user may specify which custom tags are inline