I've begun adding more blocks to the ODD file used to generate the schema, to restrict the range of values available for specific attributes. It took a lot of experimentation to figure out exactly how to do this; the only really useful documentation I was able to find was this, but even then I had to hack around at the values of the @mode attribute (they have to be "change" on the <elementSpec> tag and "replace" on the <attDef> tag, for some reason). This is what I have so far (both blocks are simplifications, as a proof of concept):

<elementSpec module="core" mode="change" ident="hi">
    <attList>
        <attDef ident="rend" mode="replace" usage="req">
            <valList type="closed">
                <valItem ident="bold">
                    <gloss>Rendered in bold.</gloss>
                </valItem>
                <valItem ident="italic">
                    <gloss>Rendered in italics</gloss>
                </valItem>
                <valItem ident="strikethrough">
                    <gloss>Rendered with a horizontal line through the middle of the text.</gloss>
                </valItem>
                <valItem ident="underline">
                    <gloss>Rendered with a line below the text.</gloss>
                </valItem>
                <valItem ident="foreign">
                    <gloss>Rendered in a distinct manner to highlight the fact that this word is not in the main language of the text.</gloss>
                </valItem>
            </valList>
        </attDef>
    </attList>
</elementSpec>

<elementSpec mode="change" ident="TEI">
    <attList>
        <attDef ident="rend" mode="replace" usage="req">
            <valList type="closed">
                <valItem ident="article">
                    <gloss>This is a full journal article.</gloss>
                </valItem>
                <valItem ident="review">
                    <gloss>This is a review of a publication.</gloss>
                </valItem>
                <valItem ident="editorial">
                    <gloss>This is editorial content.</gloss>
                </valItem>
            </valList>
        </attDef>
    </attList>
</elementSpec>