- I am reasonably confident that the answer is no. In the fluff, Siberys Dragonmarks are rare and strange beasts indeed, almost-randomly appearing, in people who previously had no mark at all.
The one candidate that came to mind, the cataclysm mage of Explorer’s Handbook, does not do so. Its Dragonmark Secret allows a 10th-level cataclysm mage to manifest any least, lesser, or greater dragonmark they choose, but even they cannot control Siberys marks.
- By fluff, certainly. Dragonmarked discusses them in its introduction to dragonmarks, as just another level of dragonmark, and though many places refer to only least, lesser, and greater marks – due to their being far more common – Siberys marks should still qualify for feats and prestige classes as would any other mark.
I would also recommend allowing them to qualify for things that specify the Least Dragonmark feat – as this is effectively shorthand for “having a true dragonmark,” typically used when a specific mark is required. The only place I would balk would be things that specifically rely on the powers of a specific mark level – Dragonmarked’s black dog prestige class makes no sense if you lack the spell-like abilities of the Least Dragonmark of Hospitality, since it expands on those abilities.
But overall it seems certain to me that “true dragonmark” is meant only to contrast with aberrant marks. This is definitely how the fluff refers to it. Unfortunately, I cannot find a mechanical statement to this effect.
- Less clear, both mechanically and fluff-wise, but I would allow it. My basis is the particular wording in the Heir of Siberys requirements:
A character who has the Aberrant Dragonmark, Least Dragonmark, Lesser Dragonmark, or Greater Dragonmark feat cannot enter this prestige class. Also, once a character has any levels in this class, he can’t adopt the dragonmark heir prestige class.
It specifically refers to entering the class with those feats, while the bit about dragonmark heir specifies that you cannot ever enter it once you have heir of Siberys levels.
Also, RAW, the Dungeon Master’s Guide specifies that prestige class requirements must be met only to take the first level of a prestige class. No general rule specifies that requirements must continue to be met, though Complete Warrior and Complete Arcane do specify this for their own classes in a way that seems to try to apply generally (but cannot, by errata rules, and also because to do so would break classes, e.g. dragon disciple).
Finally, possibly worth noting: 4e eliminated the requirement that heirs of Siberys not already hold a dragonmark. This certainly implies that there need not be a conflict between Siberys marks and their inferior brethren.
- I know Keith Baker defined how to pronounce “Cyre” (“SEER-ee”), but I have found nothing definitive for “Siberys.” Personally I go with “sib-er-EES.” Note also, however, that the pronunciation of “Cyre” is stated to be a regional thing (and other variations are offered), which may also be true for “Siberys.”