WPF supports many OpenType Font Features but Ruby isn't rendered properly, even though it is available through the Typography.Variants attribute:
<TextBlock>
<Span>
    学校<Run Typography.Variants="Ruby">がっこう</Run>
</Span>
This won't display the ruby characters as it should. I haven't found any (working) example that shows how to use "Ruby". 
One workaround (hack actually) is to use a line break as you tried to do. This is done by using the <LineBreak/> tag:
    <TextBlock >
        <Span FontFamily="Meiryo">  
            <Run FontSize="8">えみこ</Run><LineBreak/>恵美子
        </Span>
    </TextBlock>
You can put this TextBlock into a StackPanel to include it in a paragraph:
    <TextBlock>
        <Span FontFamily="Meiryo" >  This is my normal text
            <StackPanel >
                <TextBlock >
                    <Span>
                        <Run FontSize="8">えみこ</Run><LineBreak/>恵美子
                    </Span>
                </TextBlock>
            </StackPanel>
            and this is the rest of the text
        </Span>
    </TextBlock>
The result will be something like the following snippet:
 This is my normal text
<ruby>
<rb>恵美子</rb><rp>(</rp>
<rt>えみこ</rt><rp>)</rp>
</ruby>
and this is the rest of the text
 
 
The markup isn't pretty and you'll have to fiddle with offsets to get the stack panel's bottom  aligned correctly