"In general do these kinds of metta practice conflict with the annatta doctrine. I am assuming that they don't conflict as they are both fairly central in Buddhism. Given that, how are the two practices reconciled?"
I think this is something of a linguistic issue mixed with prescription/description issues -- and often discussions about "conventional" and "absolute" terms come up, etc.
For example, I can see a conflict through an ordinary/abstract approach (eg. if one disregard all that is context sensitive and assume words and meanings are fixed across the texts). Thus, the following show conflict (disregarding translation traps):
May I be well
May I be happy
May I be free
To an uninstructed run-of-the-mill person, touched by experience born of the contact of ignorance, there occur (the thoughts): 'I am,' 'I am thus,' 'I shall be,' 'I shall not be,' 'I shall be possessed of form,' 'I shall be formless,' 'I shall be percipient (conscious),' 'I shall be non-percipient,' or 'I shall be neither percipient nor non-percipient.'
-- SN 22.47:
This is a dead end. I think the issue is much like vaccine having the appearance of a contradiction: a person is cured by making her ill. The details that are important to make a better sense of it are implicit or words are too overloaded.
On a different approach, consider again an instruction to repeat the metta quote above:
If one repeats it without actively manifesting anything these words denote, one is probably exercising little more than memory by doing the repetitions.
On the other hand, if one directs the mind towards "giving" wellness, happiness and freedom, then a metta exercise is going on, in that it is expected the associated faculties to get stronger.
Now, if whatever is done while repeating, a reinforcement of a doctrine of "self" takes place, a reinforcement of wrong view happens, I guess this is the "contradiction".
So, even though the target of metta alluded to is "yourself", I believe it should be taken in the conventional sense (much as "you" and "I" are used by the Buddha), without reinforcing an illusory notion of "self". And it certainly is an important practice. For example, many struggle with self acceptance, remorse and lack of compassion towards oneself, which is not only a source of suffering but a crosscutting obstruction (may affect how one treats others, affects all kinds of things like health, humor, energy, obfuscate our views, makes it much harder to get into deeper meditation, or to jhana states, etc).