There may be no RAW, but there is a RAI method that comes close.
Savage Species presents the concept of converting racial HD and LA into racial levels, and provides guidance on the conversion process. It also presents the concept of 'younger' version of races that have weaker or not yet obtained signature abilities. Sadly, it stipulates taking all of a monster's "levels" before taking any class levels. (But read on....)
Savage Progressions takes the concept a step further and converts templates with and without HD into levels (in other words, pure LA templates too). This solidly grounds the concept of converting LA and HD into class-equivalent levels.
Savage Progressions also presents the concept of no longer having to finish a monster/template progression before taking class levels, and freely 'multiclassing' HD levels, class levels, and LA levels. The rules presented appear to be most frequently read by the community to mean that they only apply to the templates listed in the articles, rather than a global alternate ruling. (But read on....)
Unearthed Arcana presented the concept of buying off LA.
Combining these existing rules and allowing the more recent Savage Progression ruling on taking levels freely to apply to and override both Unearthed Arcana (when one is allowed to buy off LA) and Savage Species (apply the concept to all non-template levels - HD, classes, templates, etc) and applying the ruling concept in general rather than specifically (the RAI part), and we have a method of almost accomplishing what you are after.
Take one level of LA, buying it off at the third level subsequent, then take the second level of LA, buy it off three levels later, and repeat.
This interpretation of the rules provides a method that allows one to buyoff 6 levels of LA over the course of an 18 level build, at a reasonable cost in XP.
Furthermore, as KRyan noted, using Savage Species as guidelines, it is quite possible to convert LA races/templates into LA+0 using various methods and guidelines provided in Savage Species: such as spreading out the abilities over multiple levels, or weakening the abilities into lesser versions of themselves over multiple levels.
Using the rules in this fashion is technically official, as guidance is given for how to apply it globally, much in the way that using the magic item creation rules to create an item also technically makes that item official, since it follows the official rules presented for doing exactly such, or how the research rules can be used to create official spells and powers that were not printed anywhere, again, because official rules were given for accomplishing exactly that.
Please note that not everyone agrees with this interpretation of combining these rules in this fashion, but it is as close to official as one will be able to get, since it is using the official rules already presented in only minimally creative ways.
Furthermore, I quote both the authors of the original books that I have personally communicated with and discussed rules interpretations with, as well as the DMG:
(DMs)'re in charge... ...(they) are the final arbiter of the rules within the game... ...(DMs) have ultimate authority over the game mechanics, even superseding something in a rulebook.
(The original quote was speaking directly to the DM and used the words you and yours, etc..)
And that is the official rules. Since the rules themselves grant the DM authority exceeding all rulebooks, anything the DM decides IS official (in your game) as per the rules themselves.