Ten. Levels. Prestige. Class.
Important class features for the purpose of this discussion exist at:
Fourth Level (Immunity: Stunning and people with vow of nonviolence)
Seventh Level (HP damage does nothing to you in conjunction with a fourth level cleric spell).
8th level (Immunity: poison / sleep / ability damage to physical ability scores / fatigue / exhaustion / ability damage to physical ability scores / ability drain / energy drain / death from massive damage / being removed from your bone craft armor by any means)
Tenth Level (Immunity: Rogues). I'm fucking kidding. The eighth level ability means you can't remove the armor without killing you first.
So things we care about are between 4th level and 8th level (4,5,6,7 for a grand total of a window of four levels) where your DM even has the option of removing access to your class features.
So yeah, bone knight explicitly has a "you can't remove my class features short of killing my character first" class feature, and there's only a four level window where you have any class features you might be concerned about losing before getting that class feature.
You should probably talk with your DM about the fact that they deny you class features because if you can't find (seriously let's ignore immunity to stunning / nonlethal because we don't care that much about those really) time to planeshift to buy a scroll of fabricate in seven levels of advancement to craft your "this is my class feature" armor, you have bigger problems.
Cleric 6 / Bone Knight 7 (the first level you get to a bone knight class feature you really give a fuck about) is a 13th level character with 12th level cleric casting. This means sixth level spells. Plane shift is a 5th level spell. You got that an entire level ago. Since you're a cleric your holy symbol substitutes for the component of the plane you are shifting to (see: PHB). You plane shift to Sigil. Or anywhere else with a big enough marketplace. You buy a scroll of fabricate. You then drop a divine insight (see: spell compendium) with one rank in UMD and cleric charisma scores to actually use the scroll. You pass. You cast fabricate for your bonecraft armor. You then plane shift back to your initial plane. The entire thing took about a minute total.
Yes, your entire argument is invalid. The only way you would be denied class features you would care bout is DM rule zero. Please go home you are drunk.