M caron does not have a Unicode code-point. You will have to map it to the Private Use Area or perhaps to Latin Extended D.
I am wondering why you suggest the mapping of it to Latin Extended D.
Latin Extended D is part of regular Unicode.
If the M caron is mapped to Latin Extended D then a Unicode compliant software package, now or in the future, could reject the character if it is mapped to an unused position in Latin Extended D.
I would advise mapping to the Private Use Area.
If it were me doing the mapping, which it is not, I would proceed as follows.
I would start with the list of required letters.
A, a, E, e, I, i, O, o, U, u, M, m, N, n, Y, y
I would assign the following part codepoints, using X for a value on which I have yet to decide, using alphabetical order pairs of uppercase and lowercase.
U+EX0. A
U+EX1. a
U+EX2. E
U+EX3. e
U+EX4. I
U+EX5. i
U+EX6. M
U+EX7. m
U+EX8. N
U+EX9. n
U+EXA. O
U+EXB. o
U+EXC. U
U+EXD. u
U+EXE. Y
U+EXF. y
I would then make a copy of the list of required accents.
macron, vertical macron (short vertical line on top of letter), acute accent, grave accent, caron, circumflex, hook, ring, tilde, umlaut.
I would shuffle those into the order used in the combining diacritics of Unicode, using the table in the following pdf.
http://www.unicode.org/charts/PDF/U0300.pdf
That is grave, acute, circumflex, tilde, macron, umlaut (known as diaeresis), hook, ring, caron, vertical macron (known as combining vertical line above).
I would assign codes as follows.
0 grave
1 acute
2 circumflex
3 tilde
4 macron
5 diaeresis
6 hook
7 ring
8 caron
9 vertical line
I would map the special small N to U+EX8F so that it was with the other characters based around N. However, as Bhikkhu Pesala already has a small capital N at U+E2FE it might be better to use U+E2FE. My reason for saying that is that I am wondering whether mapping the small capital N also to U+EX8F for whatever value of X is chosen might upset the hinting. If it would not upset the hinting I would map also to U+EX8F for whatever value of X is chosen. This would keep the codepoints for your project all together. I am thinking that one day you might write it all up in a paper and a chart with all of the characters together would look good. That chart could show the regular Unicode codepoint for those precomposed characters which have a regular Unicode codepoint.
This would mean that I had, once I had decided on a value for X, 161 Private Use Area codes for use, though if the character were encoded in regular Unicode I would use that and not use the Private Use Area code.
For example, in the above, e acute would be U+EX31, but I would not use that as e acute has the regular Unicode assignment of U+00E9 so I would use U+00E9 for e acute.
http://www.unicode.org/charts/PDF/U0080.pdf
That and other charts are available from the following page.
http://www.unicode.org/charts/
I would then need to decide on a value for X.
I am free to choose any one hexadecimal character for that.
Now, as is well known, I already use various Private Use Area allocations for various things, so, in fact, I would avoid a clash with what I am using for something else. However, in this discussion I shall disregard my own other uses of the Private Use Area and say that I could use any of the 16 hexadecimal characters as I do not wish to influence you in your work on the basis of my experiments with other things.
However, what I would do in relation to this project is have a look at the Code2000 font and find out where there is a gap in the usage of the Private Use Area in that font.
There is no Unicode requirement to do that, but I would do it myself as I would be hoping that there would be a chance that if this project is successful that the maker of the Code2000 font might include my characters in his font at some future time. Clearly there is a better chance of that happening if the codepoints used for this project are not being used for something else in the Code2000 font at the present time.
Before deciding on the value for X, I would also have a look at which Private Use Area codepoints are already in use in the Garava font.
It is kind and generous of Bhikkhu Pesala to invite you to use his Garava font for your project.
I hope that the above notes will be helpful.
William Overington
17 November 2007