Dictionaries say すぎる in this meaning is placed:
- after the 連用形 of a verb, like 動きすぎる
- after the stem of an i-adjective, like やさしすぎる
- and after the stem of a na-adjective, like しずかすぎる.
つまらない is an adjective so I think つまらなすぎる is natural.
Generally, when すぎる is placed after ない:
- in the case of the adjective ない, it uses さ, like なさすぎる as you say
- in the case of the negative form of an i
and na-adjective, I think it uses さ, like つまらなくなさすぎる, しずかでなさすぎる.
- in the case of the negative form of a verb, すぎる is placed after な, like 動かなすぎる.
However, in the case where the stem of a verb is one character/mora, さ is often placed after な, like しなさすぎる, 見なさすぎる.
This さ is said to be an additional strength for the instability of one-character (single-mora) stems.
It is written here in detail: 「なすぎる」？「なさすぎる」？
In addition, ない is basically following.
・in the case of the adjective ない which means "not be" like 卵が無い.
・in the case of the adjective auxiliary ない which means "negative" and is mostly placed after adjective like 美しくない.
・in the case of a part of an adjective like あぶない.
・in the case of the verbal auxiliary ない which means "negative" like 動かない
However there are some persons who said the adjective auxiliary ない is same as the verbal auxiliary ない because both means "negative". In detail here. http://okwave.jp/qa/q2762974.html