I've tried chaining together an extension of the attribute with a restriction using the tag
You cannot extend/restrict either attributes or tags. You can only extend/restrict type definitions.
but it's just not working out
Ideally, you should have told us what you tried - that way, we would know what you tried, and where your understanding was slightly off.
Where can I put in a restriction to validate the content of the tag?
Actually, this is easier than you think. You put it in the same place as for a normal tag - on the tag's simple type definition. In this case, the type definition that describes the text value of the tag is the simple type that you have extended. So...
Instead of
<xs:complexType name="BType">
<xs:simpleContent>
<xs:extension base="xs:string">
...
Try:
<xs:simpleType name="EmailType">
<xs:restriction base="xs:string">
<xs:pattern value="(?i)[a-z0-9]+@[a-z0-9]+.[a-z0-9]{3}"/>
</xs:restriction>
</xs:simpleType>
<xs:complexType name="BType">
<xs:simpleContent>
<xs:extension base="EmailType">
...
(Not tested, but something like that should do the job)
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