I answered this question a while back, but unfortunately I only half-understood it, and there was some wrong information in it. After some time researching this question, and now an entire semester of a course about Japanese adjectives, I am ready to answer it again. As such, I have deleted my previous answer.
So, since しい appears to attach to the 未然形 of a verb, why with some small subset, does it attach to the -o- form of the 未然形 rather than the -a- form?
That is because しい does not attach to the 未然形 of a verb.
Rather, it attaches to the 被覆形【ひふくけい】1 of a verb. Most often, this form resembles the 未然形 -a-2, but there are a number of other examples。Some examples which still exist in modern Japanese include:
As far as I can tell (and indeed, as far as I can find in the literature), there is not a solid rule for determining the 被服形 of a verb. -a- is the most common, e.g. 喜【よろこ】ぶ→喜【よろこ】ば + しい. However, as seen in the examples above, for words in which the previous syllable ends in -o (ぼ, そ, ろ, の, etc.), there is a trend toward the 被覆形 also ending in -o.3
1 The 被覆形 is essentially a linking form. It is most often seen in reference to compound nouns, e.g. 雨【あめ】 + 雲【くも】 = 雨雲【あまぐも】 (め→ま) and 白【しろ】 + 橋【はし】 = 白橋【しらはし】 (ろ→ら）. The opposite of 被覆形 (i.e. the form when used as a standalone word) is 露出形【ろしゅつけい】.
2 This is perhaps easily understood because (at the very least after 上代), Japanese only has 5 vowels and it will thus almost certainly look like something.
3 In fact, modern 喜ばしい was also originally *喜ぼしい. e.g.
伊豫国【いよのくに】より白【しろき】祥【しるし】鹿【しか】を献奉【たてまつり】て在【あ】ればうれし よろこぼし となも見る
Note: The example above was originally printed vertically, with the bold characters having a vertical line to the left.
蜂矢真郷(2012)「上代の形容詞」『萬葉』(萬葉学会) 第212号, pp. 1-35