What is the formally correct notation of t-Bu, i-Pr, p-NO2, n-Bu, a-HCl, etc.?
In the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), single italic lower-case Latin letters are used in some stereodescriptors (such as ‘c’, ‘m’, ‘p’, ‘r’, ‘s’, and ‘t’), in descriptors for fusion sites (e.g. dibenzo[c,g]phenanthrene), in locants (‘o’, ‘m’, and ‘p’ in place of ortho, meta, and para, respectively; however, this usage is strongly discouraged), and in other structural descriptors (e.g. s-indacene).

However, single lower-case Latin letters that are used for numbering are not italicized (e.g. naphthalene-4a,8a-diol).

All these symbols are typically used in names (which are usually not typeset using MathJax/mhchem) and do not necessarily appear in formulas.

Furthermore, common abbreviations for organic structure fragments may be used in formulas, but not in text. According to the Graphical Representation Standards for Chemical Structure Diagrams (IUPAC Recommendations 2008), the permissible abbreviations include:  


isopropyl: iPr  
isobutyl: iBu  
sec-butyl: s-Bu  
tert-butyl: t-Bu


However, the ACS Style Guide recommends different abbreviations:  


isopropyl: i-Pr  
isobutyl: i-Bu  
sec-butyl: sec-Bu  
tert-butyl: t-Bu


Furthermore, authors may create their own abbreviations, but any abbreviations not included in the recommendations shall be defined clearly when they are used.

Also note (since the question mentions letters followed by an element) that several structural abbreviations in current usage are identical to an element symbol:


Ac (acetyl, actinium)  
Ar (aryl, argon)
Cm (carboxymethyl, curium)  
Nb (p-nitrobenzyl, niobium)  
Np (p-nitrophenyl, neptunium)  
Pr (propyl, praseodymium)