Great question. The short answer is that I would recommend using World Braille Usage (WBU) only for the accented characters in the alphabet and UEB for any other signs such as punctuation marks (including the UEB inverted exclamation and question marks), typeface (italic, bold underlining), etc. You do not want to mix the Spanish italics with the UEB bold, underlining, etc.
The reasoning for this recommendation: According to UEB 13.6.4 under Using Foreign Code signs, when you are using the foreign language alphabet for letters and accented letters, it is "permissible though not required to use the foreign code signs" for any other elements of the language. Also, Method 3 in the Provisional Guidance for Transcribing Foreign Language Material in UEB states that foreign language signs for accented letters are used for the foreign text, but all other signs including punctuation and font/emphasis indicators may be in UEB.
None of the guidance available to us at this time states that you are required to use either UEB or WBU for punctuation and typeface, so as long as you are consistent within your transcription, and the reader is expected to be familiar with the signs you use, you are allowed to use the Spanish punctuation and italics signs from WBU. However, because WBU lists the emphasis indicators found in other language but does not tell you how to use them (and Spanish is different than UEB), I would still follow UEB for italics, bold, etc.
I hope that clarifies. Please let me know if you have any other questions.