In The Unicode Standard, Hiragana implementation is discussed in  Chapter 18 East Asia.

Hiragana was first encoded in The Unicode Standard version 1.0. The script is encoded in the following blocks:

BlocksCharacter RangeAdded in Unicode VersionDocumentation
Hiragana 3040..309F 1.0  U3040
Kana Supplement 1B001 6.0  U1B000
Enclosed Ideographic Supplement 1F200 5.2  U1F200

Subsequent to version 1.1, the following characters have been added to the Hiragana block:

CharactersUnicode VersionDocumentation
3095..3096 3.2  n2092.pdf,  n2195.doc L2/00-098
309F 3.2  n2092.pdf,  n2195.doc L2/00-098
1F23B 9.0  

Documentation refers to  ISO Working Group and Unicode proposals

A number of documents have been submitted to the Unicode Technical Committee and WG2:

2000-03-15 Rationale for non-Kanji characters proposed by JCS committee — Japan NB ( WG2 N2092,  WG2 N2195.doc ,  L2/00-098)

2000-09-20 Ad hoc Report on Japan feedback N2257 and N2258 — T. K. Sato, Michael Everson, Ken Whistler, Asmus Freytag (WG2 N2278,  L2/00-342)

2004-05-12 Hiragana_or_Katakana value of Script property should die — John Cowan ( L2/04-160)

2004-06-17 Katakana_Or_Hiragana and UTR#29 — Mark Davis ( L2/04-152)

2009-02-15 A proposal for encoding the hentaigana characters — Jeroen Ruigrok van der Werven ( WG2 N3698,  L2/09-099)

2011-06-01 Notes on hentaigana — Japan NB ( WG2 N4091,  WG2 N4091.doc,  L2/11-229)

2015-07-22 Request for Comments on HENTAIGANA proposal (WG2 N4670; with repertoire attached) — Japan N.B. / Tetsuji Orita ( L2/15-193)

2015-09-18 Proposal of Japanese HENTAIGANA — Japan N.B. ( L2/15-239)

2015-10-31 Hentaigana List after Matsue discussion — Shuichi Tashiro ( L2/15-300)

2015-11-03 About the inclusion of standardized codepoints for Hentaigana — Tomokazu Takada, et al ( L2/15-316)

2015-11-04 About the inclusion of standardized codepoints for Hentaigana (also XLSX sheet) — Tsutomu Yada ( L2/15-318)

2015-12-09 Revised Proposal of HENTAIGANA (replaces L2/15-193; with repertoire attached; see also spreadsheet) — Japan N.B. / Tetsuji Orita ( L2/15-343)

UTC #147 Minutes ( L2/16-121) (See C.3 for decision and action items)

2015-12-09 Hentaigana proposal — Nicolas Tranter ( L2/15-334)

2015-12-16 Hentaigana Comment Document — Ken Lunde ( L2/16-053)

UTC #147 Minutes ( L2/16-121) (See C.3 for decision and action items)

2016-01-26 Hentaigana Unification Candidates — Ken Lunde ( L2/16-040)

2016-04-28 Status of hentaigana proposal — Ken Lunde ( L2/16-085)

UTC #147 Minutes ( L2/16-121) (See C.3 for decision and action items)

2016-05-10 Proposal for Japanese vowels with the voiced sound mark as named sequences — Hiroyuki Komatsu ( L2/16-133)

UTC #147 Minutes ( L2/16-121) (See B.15.5 for decision and action items)

2016-06-20 Revised Proposal of Hentaigana (with associated spreadsheet) — Japan N.B. ( L2/16-188)

Recommendations to UTC #148 August 2016 on Script Proposals — Deborah Anderson, Ken Whistler, Rick McGowan, Roozbeh Pournader, Andrew Glass, Laurentiu Iancu, and Lisa Moore ( L2/16-216) (See section 6a for recommendations)

UTC #148 Minutes ( L2/16-203) (See C.3.1 for decision and action items)

2016-06-20 Proposal to add a new character name alias to U+1B001 — Japan N.B. ( L2/16-189)

Recommendations to UTC #148 August 2016 on Script Proposals — Deborah Anderson, Ken Whistler, Rick McGowan, Roozbeh Pournader, Andrew Glass, Laurentiu Iancu, and Lisa Moore ( L2/16-216) (See section 6b for recommendations)

UTC #148 Minutes ( L2/16-203) (See B.15.2 for decision and action items)