Quail input methods with combining characters

[emacs . mexon]

Hi,

I'd like to write an input method for Chinese pinyin.  Existing pinyin 
input methods are for typing characters using pinyin, "hanyu" -> "汉语", 
not typing pinyin itself, "ha4nyu3" -> "hànyǔ".

My problem is that quail rules expect a single character (UCS codepoint) 
as the result.  But I want to insert a vowel with one or two diacritical 
marks, which is a sequence of combining characters.  Does anyone know 
how to do this, or if it's possible?

For example, I want to type a first-tone "a".  Pinyin signifies this 
with a bar over the character.  So when I type "a1", it should insert 
"a" followed by a combining macron.

(quail-defrule "a1" ?ā)

That gives an invalid read syntax, since the character after the 
question mark is really two characters.

(quail-defrule "a1" "ā")

That is accepted, but provides two options, either the unadorned "a" or 
the macron alone.

There's no way I can find precomposed characters for all cases.  For 
example, there's no precomposed character for "ǖ" ("u" + diaresis + macron).

Does anyone know if there's a way to do this, or is quail limited to 
precomposed characters?