Essentially what you are asking is can / will a Buddha do something in contradiction to the Vinaya.
In the olden days the King is not under the law he imposes. Likewise, after forbidding miracles, the Buddha announced the he will perform the Twin Miracle. A King asked you just put a rule not to display miracles and you are just going to perform one. Then the Buddha pointed out like the King being exempt from his own rules the Buddha is exempt from the Vinaya rules, i.e., Vinaya rules are only for the disciples.
Assuming you are from Sri Lanka, this is there in the old syllabus Buddhism curriculum (which was in effect in the school leaving year of 1997). I forget what year, and the exact details, but likely year 10 text book.
In this context, though a monk cannot be in a chamber of gems the Buddha can. Having said this, the chamber of gems is also not real, as it is mind made through psychic powers. In such a case I do not think there is an issue for a monk prior to the forbidding of psychic power display. As in this case there were not witnesses hence, if a does this, in private it will still not break the rule of touching gold (it is not real - it is just a mind projected image). This is just my opinion, there might be other vinaya rules in play.
In addition this is not the traditional account:
The stone chamber and the stone seat (length 6.5 feet width 3 feet) made and offered to Buddha by King Muchalinda using the highest technology prevailed to that date are still exist there. Earlier it was called Ruwan Geya
More orthodox account of it is:
The Golden Bridge
In the third week, the Buddha saw through his mind’s eye that the devas in the heavens were not sure whether he had attained enlightenment or not. To prove his enlightenment the Buddha created a golden bridge in the air and walked up and down it for a whole week.
The Jewelled Chamber
In the fourth week, he created a beautiful jewelled chamber and sitting inside it meditated on what was later known as the "Detailed Teaching" (Abhidhamma). His mind and body were so purified that six coloured rays came out of his body — blue, yellow, red, white, orange and a mixture of these five. Today these six colours make up the Buddhist flag. Each colour represented one noble quality of the Buddha: yellow for holiness, white for purity, blue for confidence, red for wisdom and orange for desirelessness. The mixed colour represented all these noble qualities.
The Mucalinda Tree
The Buddha then went and meditated at the foot of a mucalinda tree. It began to rain heavily and a huge king cobra came out and coiled his body seven times around the Buddha to keep him warm and placed his hood over the Buddha’s head to protect him from the rain. After seven days the rain stopped and the snake changed into a young man who paid his respects to the Buddha. The Buddha then said:
"Happy are they who are contented. Happiness is for those who hear and know the truth. Happy are they who have good will in this world towards all sentient beings. Happy are they who have no attachments and have passed beyond sense-desires. The disappearance of the word "I AM " is indeed the highest happiness."
Life of the Buddha - Chapter 18: Seven Weeks After Enlightenment
The Jewelle Chamber is separate from the time under the Mucalinda Tree.